I do a microwave veg curry sometimes.
Frozen veg, rice, water and curry paste. Cover, puncture the plastic film, and zap on high for 10mins, stir and repeat.
Also, while frozen veg is usually cheaper I think I’m right in saying that pre chopped onions are far more expensive than buying whole onions. Do you have a food processor, or could you just cut them bigger?
Agree with eggs for a cheap filling meal. Omelettes, scrambled eggs on toast, spiced up with small bits of chorizo or spices if you want, I like eggs with black pepper. Mushrooms on toast is delicious, IMO. Also, things like sausages/bacon/chicken/fish go much further if you slice or dice them. So the frozen fish with curry paste suggested, you could use 1 piece for both of you mixed with veg and rice.
Tonight I did pasta for four adults, 3 rashers of bacon chopped small with onion, courgette and peppers and a can of tomatoes. You could add herbs like oregano, I didn’t have any so used black pepper instead.
You can add almost anything to a quick tomatoe sauce for pasta. Grated veg, canned tuna, minced beef, chicken thighs - much cheaper than chicken breast- olives. Whatever takes your fancy, and the same for rice based dishes. Upping the veg contents makes it easier to cut the carb portion down.