Preheat oven to 400 degrees.
Line a pizza pan or baking sheet with parchment.
If using homemade dough, roll it into a circle approximately the circumference of the pizza pan or a rectangle to fit the baking sheet. Trim any edges. For frozen dough, shape it to fit the pizza pan. Store the crust in the refrigerator while you prepare the topping.
Peel the onions and slice into thin rounds, approximately 1/4-1/3-inch thickness.
Sprinkle the crust with the cheese, going all the way to the edges.
Top with the thyme, sprinkled evenly across the crust.
Starting at the center edge of the crust, place the onion rounds in rows, slightly overlapping each onion. (Working from the center to the left, then to the right makes for easy placement of the rounds, if you’re using a circular pan.) You can start from the edge or go diagonally if you’re making a rectangle.
Lightly brush all the onions with milk then dot with the cubes of butter.
Bake for 40 mins or until the tart is golden and onions begin to brown. If the edges of the tart start to brown during baking, cover them with foil.
Serve hot or at room temperature.