Mushrooms Stuffed with Tomatoes and Olives

Preheat oven to 375°F. Spread walnuts on baking sheet, and toast in oven until lightly browned and fragrant, about 6 minutes. Set aside to cool. Reduce oven temperature to 350°F.

Put sun-dried tomatoes into small bowl, and cover with hot water. Cover with plastic wrap, and let stand until tomatoes are plump, about 30 minutes. Drain well, and chop. Set aside.

Meanwhile, steam potato quarters until tender, about 15 minutes. Transfer to small bowl, and let cool slightly. Mash potato quarters with fork, then add milk and stir until smooth. Set aside. Finely chop enough mushroom stems to equal 1/3 cup, and set aside. Reserve remaining stems for another use; store them in refrigerator up to 3 days.

In large skillet, heat oil over medium heat. Add onion, garlic and red pepper flakes, and cook, stirring often, until onion begins to brown, about 10 minutes. Add sun-dried tomatoes, olives, and chopped mushroom stems, and cook, stirring often, scraping bottom of pan occasionally, 2 to 3 minutes.

In medium bowl, mix all ingredients except mushroom caps. Mixture should be moist but hold its shape well. Add small amounts of water if mixture is too dry. Season with salt to taste. (Filling can be made ahead and refrigerated up to 2 days. Return to room temperature, and moisten with additional water if necessary before proceeding.)

Coat shallow baking dish with nonstick cooking spray. Spoon filling into mushroom caps, mounding slightly. Arrange mushrooms in prepared baking dish. (Mushrooms can be prepared up to this point and refrigerated, covered, for 1 day.)

Bake until mushrooms are tender when pierced with fork and filling is heated through, about 20 minutes. Serve warm or at room temperature.