Leek and Asparagus Frittata

INGREDIENTS

• 1/2 teaspoon salt
• 1 cup sliced stemmed shiitake mushrooms
• 1 12-ounce bunch thin asparagus, trimmed, cut on diagonal into 1-inch pieces (about 2 1/2 cups)
• 8 large omega-3 eggs
• 1 cup diced Fontina cheese, divided
• 1/2 teaspoon ground black pepper
• 1/4 cup grated Parmesan cheese
• 2 tablespoons (1/4 stick) butter
• 1 cup chopped leeks (white and pale green parts only)