Of course, as an Italian, my first definition of "simple meal" is a pasta dish with a simple tomato sauce. Another really simple yet amazing recipe is "cacio e pepe": a dish of spaghetti (but bucatini are best) with melted butter, fresh grated cheese (I prefer pecorino romano for this, but really, any hard cheese will do), and a liberal amount of black pepper.
Yum. I use Greek yogurt on pasta, too, and it's really good. Still playing around with the seasoning.
My simplest meal is what I call Instant Chili: throw a can of low-sodium black beans and a can of diced tomatoes (both drained) into a microwaveable bowl; season with cayenne pepper, garlic, onion flakes, oregano, etc; nuke for three minutes. Works with chickpeas instead of black beans, too.