12 medium-size mushrooms caps
1 Tablespoon olive oil
1 Tablespoon sweet butter
1/2 cup finely chopped yellow onion
2 Tablespoon coarsely chopped walnuts
1 garlic clove, peeled and minced
5 ounces frozen spinach, thoroughly defrosted and squeezed dry
1 ounce feta cheese, crumbled
1 ounce Gruyere cheese, crumbled
2 Tablespoons minced fresh dill
salt and freshly ground black pepper to taste
1. Remove stems from mushroom and save for another use. Wipe the mushroom caps with a damp cloth or paper towel and set aside. Heat the olive oil and butter together in a small skillet. Add the onion and cook over medium heat, covered, until tender and lightly colored, about 25 minutes. Preheat oven to 400 degrees.
2. Add walnuts and garlic to onion and cook for another minute. Add spinach and cook for another 5 minutes, stirring constantly. Remove from heat and cool slightly. Stir in cheeses, dill, and salt and pepper to taste.
3. Arrange the mushrooms, cavity side up, in a baking dish. Divide the spinach and walnut mixture evenly among the mushroom caps. Set 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.