Preheat oven to 350° F. Begin by mixing eggs and milk very well in a mixing bowl. Add the salt and pepper. To make the first frittata serving: Heat 3/4 tablespoon butter and 3/4 tablespoon olive oil in a saute pan over medium-high heat. Once heated, place 1/3 the amount of sausage, 1 1/2 tablespoon scallion, 1/4 cup red onion, 1 teaspoon garlic and 1/4 cup tomato into the saute pan. Cook ingredients for 4 minutes then add 1/3 the amount egg mixture to the pan. As soon as the egg mixture has been added to the pan, remove the pan from the heat and place in oven and cook for 3 minutes or until the top portion is cooked. To remove from pan, gently tilt the pan and slide the frittata onto the plate. Once plated, drizzle the balsamic vinegar evenly across the top. Repeat cooking steps for second and third frittata portions.