Slice tomatoes and place onto towels to pull out excess moisture.
Sauté onion in skillet with butter until soft and golden, approximately 10 to 12 minutes.
Evenly line ungreased 9-inch pie pan or tart pan with pie crust. Bake crust at 425 F for 8 minutes on bottom or middle shelf of oven. Let cool.
Spread alternate layers of onion, cheese, and tomatoes on top of pie crust, reserving ½ cup of cheese. Combine eggs, milk, salt, and pepper. Beat lightly, then pour over tart. Top with remaining shredded cheese.
Cover loosely with aluminum foil and bake for 30 minutes, removing foil for last 5 minutes to brown cheese. Serve hot or at room temperature.
Per serving: 293 calories, 18 grams fat (8 grams saturated fat), 21 grams total carbs, 2 grams fiber, 12 grams protein