portobellos stuffed with caramelized onions and manchego

These mushrooms look like mini-pizzas. They make a great dinner-party first course or, served with a green salad, a luncheon main course. For a cocktail party, make them with small caps.

MAKE AHEAD: Prep the onion stuffing and cheese topping up to 2 days in advance. The mushrooms are best baked to order, but they can be assembled and refrigerated, covered, several hours in advance. Bring them to room temperature before baking.

4 first-course servings (or 24 hors d'oeuvres using smaller mushrooms)

Ingredients:

For the mushrooms

• 4 large (4 1/2-inch diameter) portobello mushrooms, or 24 small "baby bellas" (2 1/2-inch diameter), cleaned, gills and stems removed
• 2 tablespoons olive oil
• 2 medium (1 1/2 pounds) yellow onions, cut in half, then into 1/4-inch slices
• Salt
• Freshly ground black pepper
• Leaves from 1/2 small bunch of thyme (2 tablespoons)
• 1/2 teaspoon garlic powder

For the topping

• 2 ounces Manchego cheese, finely grated (2/3 cup)
• 1/4 cup regular or low-fat mayonnaise (do not use nonfat)

Directions:

For the mushrooms: Line a baking sheet with aluminum foil, then coat it lightly with nonstick cooking oil spray. Arrange the mushrooms on the foil stem sides up.
Heat the oil in a large saute pan over medium-high heat until the oil shimmers. Add the onions and stir lightly to coat; season with salt and pepper. Cook for 10 to 15 minutes, stirring often, until the onions' moisture has evaporated and the onions begin to sizzle and brown. Reduce the heat to medium and cook for 15 to 20 minutes, stirring often, until the onions are deep brown and caramelized; adjust the heat as needed so they do not burn.
Transfer to a medium bowl. Add the thyme and garlic powder; add salt and pepper as needed, keeping in mind that the mushrooms themselves will not be seasoned. The yield will be 1 1/2 cups.
Spread all of the onion mixture on the insides of the large mushroom caps, or fill the smaller caps with it.
Preheat the oven to 400 degrees.
For the topping: Combine the cheese and mayonnaise in a small bowl. The yield will be 1/3 cup. Divide the topping evenly among the 4 large caps or the smaller caps.
Bake for 20 minutes (15 minutes for the smaller mushrooms), until the topping is golden brown and bubbling. Blot the caps on paper towels before serving.