Preheat oven to 400°F

Bake pie crust in preheated oven until firm, about 8 minutes. Remove crust from oven and set aside. Reduce oven heat to 350°F

Heat olive oil in a large skillet over medium heat. Cook and stir onion, green bell pepper, mushrooms, and spinach in hot oil until soft, 5 to 7 minutes. Remove vegetables from skillet and set aside.

Whisk eggs, milk, salt, and pepper together in a small bowl

Spread 1 cup Colby-Monterey Jack cheese in the bottom of pie crust. Layer vegetable mixture over the cheese and top with the tomatoes. Pour egg mixture into pie shell. Sprinkle remaining 1/2 cup cheese atop the quiche.

Bake in preheated oven until a knife inserted near the center comes out clean, 40 to 45 minutes. Cool 5 minutes before serving