My ds is 16 months, most lunchtimes he has cream cheese sandwiches, or bread and butter with cubes of cheese, or ham, plus fruit & yoghurt.
Sometimes he has baked beans (the low sugar low salt heinz ones), scrambled egg or omelette, with toast - these all take minutes to prepare.
For tea he has his main meal, usually casserole, fish or shepherds pie, or similar, usually from a batch I have frozen, but if it hasn't got potatoes in it, I do him fresh pasta or couscous to go with it, and some fresh / frozen veg.
You could make pasta bakes / lasagne, and not mash it, so that you freeze it as you would for an adult, then your MIL can lightly mash it for her if necessary (probably won't be), & the same with shepherds pie etc.
What would your MIL be eating, do they eat at the same time?