In a large ovenproof skillet, heat olive oil over medium-high heat. Sauté sausage until cooked through, about 5 minutes. Add peppers and spinach to the skillet. Cook together until spinach is wilted, about 2-3 minutes.
In a mixing bowl, whisk together eggs, milk, salt and pepper. Stir in shredded cheese. Pour egg mixture into the skillet. Stir immediately to distribute ingredients. Cook frittata, lifting the edges to allow uncooked egg underneath. Once bottom and edges are firm and top is loose, about 5-7 minutes, remove from stovetop.
Top frittata with fresh mozzarella balls, then transfer to the oven and bake for 8-10 minutes, or until cooked through. If desired, broil frittata for 2-4 minutes to brown or melt extra cheese on top.
Remove from heat. Using a rubber spatula, loosen the frittata from skillet and slide the frittata onto a cutting board. Cut the frittata into wedges and serve.