I'm wondering how best to deal with my mum who has had dementia for 3 years now, and keeps trying to prepare food for my deceased dad.

Just let her get on with it. It is wasting a bit of food but if she is not distressed by it still being there and her throwing it away then just let her carry on and don't let it stress you. You could try telling her he was away for a while on work and seeing if that stopped her without causing problems rather than keep explaining he is deceased, then the next time just remind her he has had to go away for a few days - don't know if this would help, just an idea.

Classic. Margaret Forster wrote a book called "Have The Men Had Enough?" which describes exactly this common aspect of Alzheimer's Disease.

You couldn't eat the sandwiches? Don't try to stop her making them. Or, if she wants to be involved in food prep, how about finding her something to do that's actually useful?

Divert her into an new habit. Maybe have her set the table for those folks present - you and her. It might prompt her to make only enough food for 2. As for leftovers, maybe package them up before the end of the night - so they don't go bad. Label them "lunch."

it sounds very tough.

i just want to say: how sweet of your mother.

i understand it's problematic, but it's so loving of your mother.

I've heard this happen to a distant relative in India (I'm from there originally). The elderly lady would make rice for her husband daily even tho' he was no more. But someone in the household would eat the rice.

Sorry I can’t offer a solution. I can only offer support from having had a similar situation. For her final year with us my mother was certain she was hosting a luncheon. (she is now in care) She would set the dining room table with crystal and china. The problem? As soon as the sun poked up on the horizon, she’d pound on my bedroom door, furious that I wasn’t already in the kitchen cooking and baking, as I was to cater said luncheon. Left me exhausted. (and a crappy way to start the day) We tried resperidone to break the obsessive cycle. Now sure if it helped because (like so many others on this site) we had a crisis that resulted in an emergency placement a week into the prescription.

For 15+ years she’d been inviting people to her home and parading them into my house (we were neighbours) at mealtimes. Yes, she was difficult and demanding, with a long list of her own boundaries but no respect for mine. But that’s another topic.

Nothing I said or did would dissuade her from this fixation. I hope you survive the odd obsessive phases of her broken brain.

Suggest leaving a piece of fruit, and a type of bread or roll/ crackers/ cookie for Dad. A cup of tea, or coffee. A little sandwich.
As long as she's throwing the food out ( or you are), it gives her a connection. It is a sign of love, and caring. Even with the dementia.
Is this causing an economic hardship? Is there food for her and your house hold?
In some faith traditions, it is a real thing to leave a snack for the deceased.
The hardest conversation I had with my mom was "where are my sisters, are they here?" She didn't loose it when I told the truth.
It is hard on you, when you are having to answer this heart breaking question, and deal with this daily.

Can you wrap the sandwiches and put them in the fridge when she isn't looking or has gone to bed? Is there a neighbor that would appreciate the food? A family with young kids that could eat the sandwiches? An elderly person living alone?

What about including her in dinner preparations? Then make up a plate for dad that he can microwave. Then this can be used as lunch for the next day.

With food prices what they are, I am sure you could find someone in the neighborhood that would appreciate the extra food.

Just let her keep making sandwiches for her deceased husband and then throwing them away the next morning, uneaten. There is no way to 'properly' deal with this matter, really. You can tell mom he's deceased until you're blue in the face, but the words won't sink in right now. So don't bother. This phase may pass eventually, and then she'll become obsessed with something else you'll try to distract her from. That's the nature of dementia, which is a hideous thing to be afflicted with. My mother who has advanced dementia herself is now obsessed with her dead relatives who she's convinced have 'abandoned her' rather than died. I've told her everything from they've died to they're busy to they don't have telephones; nothing works long term. The next day or hour she's back to saying the same thing; that her siblings and her parents have 'abandoned' her, which is sadder than having her know they've died, in reality. At least there's a finality to death that closes the subject, you know? With abandonment, she feels that pain daily! Horrible.

When telling them the truth causes more problems than using a fiblet, then we have to get even more creative with our stories. Again, there's no 'right or wrong' way to deal with a demented person in this area. Do whatever you have to do to help your mom to the best of your ability.

I hope this phase passes for your mom soon.

Best of luck.