Spray an 8-inch broiler-safe baking dish with cooking spray, and set aside.

Melt butter in a large saucepan over medium heat. Add garlic and crushed red pepper and cook, stirring often, until garlic is lightly browned and aromatic.

Add rice, and cook, stirring occasionally, until lightly toasted.

Stir in stock, salt, pepper, and 1 1/2 cups marinara sauce. Bring to a boil over medium heat, reduce heat to medium-low, and simmer, covered, stirring every few minutes, until rice is almost fully cooked through.

Preheat the oven to 350 degrees F.

Remove from heat, stir in Parmesan cheese and remaining 1/2 cup marinara, until evenly combined.

Transfer rice mixture to the prepared baking dish, spreading it into an even layer.

Top evenly with pepperoni and mozzarella.

Bake in the preheated oven until cheese is melted, about 10 minutes.

With baking dish still in the oven, increase oven temperature to broil; broil until cheese is just golden brown.