A different version of rarebit: grated cheese, an egg, mustard or ketchup (or both!). Beat together and spread on toast, then broil until cooked through. You can add chopped tomatoes and onions when you've got them. I've also made it as a casserole with stale bread cubes: spread alternate layers of bread cubes and grated cheese in large casserole dish, beat 3 to 6 eggs with half a cup of milk and a half-teaspoon mustard and pour over, bake at 350F for half an hour or so.

German pancake: beat 4 eggs well, slowly add 2/3 cup flour, then stir in 2/3 cup milk. Melt some butter in a cast-iron skillet or similar baking dish, pour batter in, bake at 400F for 11 minutes (depending on your oven). Top with sauteed apples, or lemon juice with powdered sugar, or think of it as yorkshire pud and top it with brown gravy with bits of leftover beef in.

I tend to think of dairy and eggs when I think of cheap protein, but whole chickens or chicken thighs on sale can be very cheap, and you can eat the meat one meal, casserole the rest, and make soup from the bones.