Egg(s) on toast (usually fried or scrambled), chuck a couple of halved tomatoes under the grill for veg if wanted or fry some mushrooms.
Omelette. I like both the folded sort and also a flat Spanish-style one with whatever is in the fridge.
Pasta with any cooked leftover meat chopped small or a pack of small prawns, fried onion, butter, lemon juice, frozen peas/sweetcorn, chilli if liked, suitable herbs from freezer. Heat 'sauce' ingredients in a pan and add cooked pasta, mix well.
Quinoa salad - perfectly nice just with grated carrot, chopped tomatoes/cucumber and onion/spring onion plus any other suitable veg (frozen peas are good, red cabbage is really nice in this too). Chop veg, add olive oil and vinegar/lemon juice/seasoning, cook quinoa, mix together. Some feta crumbled into it or a tin of chickpeas adds protein. You can also do this with couscous or rice.
Tinned hicken in white sauce, hot buttery rice, cooked frozen peas. LOVE THIS. Guity pleasure.
Tinned salmon curry with a wrap or microwave rice. Fry plenty of onions, add curry powder, add salmon, break up and stir, add a little water and simmer for a bit. This is from my West Indian grandma and it is surprisingly good. You can add frozen veg to this before simmering too. I guess it would probably work with tuna too.
Fried rice always a winner here as well!