This is great for this week and freezing for another. Use left over beans and rice (cook extra earlier in the week when making other dishes). If you're using canned, add extra seasoning (leftovers would be seasoned from the previous meal).
Stale bread crumbled seems to work fine without affecting the flavor.
Season any way you like. Garlic onion is popular, I've listed a taco type here. Think about how the leftovers you are using are already seasoned any play that up. Fiery hot peppers are a great addition too!
Freeze first if grilling. Transferring from freezer paper directly to pan has prevented any breaking problems. Wait until they are partway cooked before flipping, and you should have success with black bean burgers that hold together!