Nonstick vegetable oil spray
1 12-ounce bag baby spinach
1 tablespoon olive oil
1 cup chopped onion
8 ounces mild Italian sausages, casings removed, sausage broken into 1-inch pieces
8 large eggs
1/4 cup heavy whipping cream
1/2 teaspoon salt
1/2 teaspoon freshly ground black pepper
1 cup crumbled feta cheese (about 4 1/2 ounces)
Preheat oven to 325°F. Spray 8 x 8 x 2-inch glass baking dish with nonstick spray. Bring large pot of salted water to boil. Add spinach and cook just until wilted. Drain the spinach and chop, then place in kitchen towel and squeeze dry. Set aside.
Heat oil in large nonstick skillet over medium-high heat. Add onion to skillet and sauté until soft, 4 to 5 minutes. Add sausage and sauté until brown and cooked through, breaking up with fork, 5 to 7 minutes. Remove from heat and cool.
Whisk eggs, cream, salt, and pepper in large bowl to blend. Add the spinach and cooled sausage mixture, then feta; stir to blend. Transfer mixture to prepared baking dish.
Bake frittata until set in center, 40 to 50 minutes. Transfer baking dish to rack and cool frittata 15 to 20 minutes. Place platter atop dish with frittata. Using oven mitts, hold baking dish and platter firmly together and invert frittata onto platter; place another platter atop frittata and invert again so that frittata is right side up. Cut frittata into 20 pieces.