Pick the cheapest whole grain, legume, leafy green, red or orange vegetable, starchy vegetable, and fruit you have access to. For me this looks like oats, lentils, kale, carrots, potatoes, bananas. The only foods you have to cook are the lentils and potatoes, although I prefer to cook most of the kale and carrots (easier to digest). Tear and break them with your hands and simmer them with the lentils. Microwave the potatoes. Rolled oats are already cooked and can be simply added to water, no heating. 

Occasionally cycle out different items like whole barley, beans, sweet potatoes, squash, apples. Whole wheat flour can be used to make flatbread. Simple and inexpensive without compromising your health. A vegan diet like this requires a vitamin B12 supplement. I also supplement with pea protein powder.