These two meals kept me more or less healthy as a student, even through "omg I have 10 eur to last me 2 weeks".

1. Lentils and rice form complete proteins when eaten together. Lentils are a staple, and very cheap. They should be bought dry. Look for Indian recipes for daal for inspiration, or just cook with some broth and fry up some garlic and onion to throw in at the end.
2. Fill up a casserole with potatoes to boil, but leave some room on top, use a lid and don't fill up water so it completely covers your taters. Mackerel wrapped in aluminium foil with some aromatics inside like bay leaves, lemon slices if you're not a fan of fish. Place the fish on top of the potatoes when there's 25 min left on them. This dish also consumes very little electricity, but most importantly it will provide you with plenty of omega 3, vitamin d and all macros you need. Super important for those who live places where you get little sunshine through the winter months!