I am still not used to cooking for the three of us, ds1 and dd have mostly left home, so I always have leftovers!
I also always batch cooked so the freezer is usually crammed!
Make lots of bolognese sauce, you can use that for spag bol, or lasagne
Stews and casseroles, make lots it can double as pie filling ,under potato like a cottage pie, or fake pastry lids from ready roll pastry
Poach a whole chicken...strip off all the meat. Last time I did this I made a chicken, leek and white sauce pie filling, used half and froze half, all I had to do for tonight's supper was roll out pastry
If I find chicken breast in the reduced I make hunters chicken (slice open like a book, bash them flat, roll up a stick of cheese and the wrap the lot in a couple of slices of bacon. Bag them individually and all you have to do is defrost and shove them in the oven with sauce
Mash .....make loads and freeze it, now imo it doesn't reheat nicely as plain mash, but let it defrost and add in some extra cheese and sliced spring onions, reheat it in the oven and it's gorgeous.
Or I often make loads of savoury mince , and put it in the freezer...with your spare mash you are good to go for a cottage pie.