Mushrooms Stuffed with Walnuts and Cheese

12 medium-sized mushroom caps
1 tablespoon olive oil
1 tablespoon sweet butter
½ cup finely chopped yellow onion
2 tablespoons coarsely chopped walnuts
1 garlic clove, peeled and minced
5 ounces frozen chopped spinach, thoroughly defrosted and squeezed dry
1 ounce feta cheese, crumbled
1 ounce gruyere cheese, crumbled
2 tablespoons minced fresh dill
salt and pepper to taste

1.Remove stems from mushroom caps and save for another use. Wipe the casps with a damp cloth or paper towel and set aside.
2.Heat oil and butter together in small skillet. Add the onion and cook over medium heat, covered, until tender and lightly colored, about 25 minutes.
3. Prejhaet oven to 400.
4.Add walnuts and garlic to onion and cook for another minute. Add the spinach and cook for another 5 minutes, stirring constantly. Remove from heat and cool slightly. Stir in the cheeses, dill, and salt and pepper to taste.
5.Arrange the mushrooms, cavity side up, in a baking dish. Divide the spinach and walnut mixture evenly among the mushroom caps.
6.Set the baking dish in the upper third of the oven. Bake for 8 to 10 minutes, or until filling is browned and the mushrooms are thoroughly heated through. Serve immediately.

12 mushrooms, or 3-4 servings