Here we take the elements of a vegetarian lasagna filling--ricotta, spinach and parmesan cheese--and nestle them into roasted portobello mushroom caps. The recipe works best with very large portobello caps; if you can only find smaller ones, buy one or two extra and divide the filling among all the caps. Serve with a tossed salad and a whole-wheat dinner roll or spaghetti tossed with marinara sauce.