Preheat the oven to 350°F (180°C). Remove the stems from the mushrooms and chop them finely. Melt the butter in a large skillet over medium heat. Add the chopped mushroom stems, shallots, garlic and zucchini, and cook for 5 minutes, stirring occasionally.
Remove the pan from the heat and stir in the chopped spinach, bread crumbs, hazelnuts, parsley, and salt and pepper to taste.
Place the mushroom caps, hollow side up, in a single layer in a lightly sprayed shallow ovenproof dish or on a lightly sprayed baking tray. Heap some of the shallot and zucchini mixture into each mushroom cap and sprinkle the Parmesan cheese over the top. (The mushrooms can be prepared 2 to 3 hours ahead and kept, covered with plastic wrap, in the refrigerator.)
Bake until the mushrooms are tender and the cheese has melted, about 15 minutes. Serve warm, on a bed of spinach leaves, if desired.
Recipe Notes
(one mushroom) provides: 53 calories, 36 calories from fat, 4g fat, 1 gram saturated fat, 2 grams protein