Caramelized Onion-Grape Tomato Tart

This is my version of a recipe from The Pioneer Woman. Cooked crumbled bacon would be a terrific addition! (I omitted it only because we had this tart with grilled rib-eyes topped with lemon-blue cheese butter. Seemed a little over the top!)
1 round pie dough (homemade or purchased)
4 Tbsp. butter
1 red onion, cut in half and thinly sliced
Salt and pepper
4 oz. shredded Italian cheese blend
4 oz. shredded Swiss cheese
1 pint grape tomatoes, cut in half

1. Roll out pie dough, and shape to fit a half-sheet pan.
2. Meanwhile, melt butter over low heat in a large skillet; stir in sliced onion and salt and pepper to taste. Cover and cook, stirring occasionally, for 20 minutes or until onions are very soft.
3. Sprinkle cheeses over top of pie dough; top with caramelized onions and grape tomato halves. Use a pastry brush to spread remaining butter in onion pan around edges of crust.
4. Bake at 425° for 15 minutes or until crust is golden and cheese melts. Serve immediately or at room temperature.