At school I was taught to make meatloaf with mince and sausage meat which I serve sliced with roast potatoes, Yorkshire puddings and veg. It’s a very cheap roast which goes a long way. I also make a version with breadcrumbs and cheap sausage meat which I serve cold, sliced in sandwiches which makes a change from cheese and is cheaper than ham.
Swap beef mince for pork, turkey or chicken, or quorn if you can eat it.
Buy liver, kidney and other offal from a butcher and get them to mince it and add some of that in to your mince.
As others have said, red lentils disappear into a ragù, as do oats.
Tinned fish is still very economical, mackerel or sardines. Stir through pasta, serve on naice bread, jacket potatoes, etc.
Always serve bread, flatbreads etc on the side as appropriate to the meal. Especially if you make home made. They can be frozen.
Serve extra potatoes, rice, or pasta. Have a bean based side dish available and see if, after all, they do like it alongside their (gradually reducing) meat portion.
Savoury bread and butter pudding with maybe some bacon chopped up through it.
Macaroni cheese.