Cook ground pork sausage and chopped onion in a skillet over medium-high heat, stirring until sausage crumbles and is no longer pink; drain.
Toss together sausage mixture, broccoli, and 1/2 cup cheese blend. Spoon into a lightly greased 11- x 7-inch baking dish.
Stir together 1/2 cup cheese blend, eggs, and next 4 ingredients; pour over broccoli mixture.
Sprinkle with remaining 1/2 cup cheese blend. Arrange tomato on top.
Bake at 350° for 30 minutes; uncover and bake 15 more minutes. Let stand 10 minutes before serving.