Heat 1 teaspoon olive oil in a skillet over medium heat; cook and stir pancetta until fat has been released, 3 to 5 minutes. Transfer pancetta to a paper towel-lined plate.
Pour remaining 5 teaspoons olive oil into the same skillet; cook and stir garlic until fragrant, about 20 seconds. Add Brussels sprouts to garlic; cook, stirring constantly, until sprouts begin to brown, 5 to 10 minutes. Transfer sprouts and garlic to a bowl and crumble cooled pancetta over the mixture. Add mozzarella cheese and fennel seed; toss to coat. Place pizza crust onto a baking sheet and spread Brussels sprouts mixture atop pizza crust.
Bake in the preheated oven until cheese is melted and bubbling, about 10 minutes.
