That's easy - oatmeal! Extremely nutritious and it never tastes bad, because it never tastes at all. Also extraordinarily cheap if one buys rolled oats in quantity rather than those tiny little packets adulterated with sugar and spice.

I am about to sit down to my typical breakfast of oatmeal, milk, raisins, and various nuts - two minutes in the microwave and it is quite tasty. At any rate,probably a lot better than the Scottish warrior's meal, which was reputed to be a handful of oats dipped in a passing creek - no microwave back in those days!