@BogstandardBelle - my sister and I used to get sent to stay with a friend of my mum for a couple of weeks in the summer holidays - and she had the same attitude towards leftovers as your MIL. They'd all be carefully kept in the fridge, and then, on Friday, she'd do 'Scraps Dinner and Scraps Pudding' to finish them all up before the weekly shop on Saturday.
Basically you'd get a bit of everything savoury, with veg or salad to bulk it out, followed by a bit of everything sweet.
She outdid herself one year, when she and her dd ended up coming back to stay with us - we all travelled on the coach, and she packed a picnic, comprised of the leftovers from tea the previous day. Tea had been slices of bread and butter with different toppings - including marmite, jam, peanut butter and chocolate spread - open sandwiches, basically.
She had turned these into actual sandwiches by putting slices together, based on what the filling looked like. Peanut butter was fine - it didn't look like anything else, so we got peanut butter sandwiches. Two different flavours of jam was interesting but not unpleasant.
Chocolate spread and marmite was ... there are no words for how disgusting it was. I get travel sick on coaches, and this picnic did not help!
Before the first time we went to stay, she asked my mum if there was anything dsis and I did not eat, and mum told her we both loathed liver - so the very first picnic she made included liver pate sandwiches . We were both too well dragged up to complain/refuse, so we held our breath and ate them - and she decided, based on this, that mum was wrong, so we got lots of liver pate sandwiches - which was bad enough - but then one day she made liver casserole. This broke dsis - she refused (politely) to eat it, but I held my breath and forced it down, because I didn't dare not. I did such a good job that she offered me seconds - and I finally got to say No thank you.
It is only very recently I have been able to face liver at all - dh likes it and occasionally buys it and cooks it for his lunch, but he cooks it very well, and I can eat a little bit and enjoy it.
Talking of liver - I remember working as a Student Nurse, and having to help elderly patients with their meals. One lady basically needed pureed food - she could hardly manage to chew at all, and so couldn't cope with anything bigger than mince. One day, the lunch I was given to feed her was a slab of liver with mash and cabbage, and I had to spend ages mincing tough ox liver with a normal knife and fork, until it was small enough to mix with the mash, so she could swallow it (and so the strong taste was somewhat disguised). Poor woman.