Arrange a rack at the top of the oven and heat the oven to 425°F.
Line a baking sheet with aluminum foil. Split 2 bagels and place cut-side up on the baking sheet. Brush with 2 teaspoons olive oil. Bake until golden-brown, 5 to 7 minutes. Remove from the oven and switch the oven to broil.
Spread 2 tablespoons pizza or marinara sauce on each bagel half, spreading to the very edges. Top each bagel half with a heaping ⅓ cup shredded mozzarella cheese. Add toppings as desired.
Return the baking sheet to the oven and broil until the cheese is melted and browned in spots, 2 to 5 minutes. Garnish with fresh basil, grated Parmesan cheese, and/or red pepper flakes if desired.
