I've had chicken breast and broccoli for breakfast. I've had omelets for dinner. Nutritious food isn't time-specific.
BY far the biggest improvement most people could see from a cost basis is learning to cook and then actually cooking. The wife and I cook 2-3 times a week, make enough to last a few days and reheat foods as needed.
BY far the biggest improvement most people could see form a nutritional basis is the removal of food additives and taste enhancers that are designed to make food palatable for consumption while enhancing shelf-life or allowing the use of substitute ingredients that cost less. (Modified food starch, I'm looking at you!)