I like the "build a toaster starting with raw materials" angle. But if the real goal is "toast" and the real starting point is "nature", I would grab a bunch of sticks, start a fire, and hold a piece of bread over the fire like a marshmallow. That reduces the problem to that of making dough and baking bread, of which the most difficult problems I see would be (a) grinding the wheat and (b) leavening the bread.