Halve peppers lengthwise and remove stem, pulp, and seeds; place cut-side up on a baking sheet. Lay 1 slice turkey inside each pepper. Sprinkle chopped onion evenly over turkey. Drizzle 1 teaspoon pesto inside each pepper half. Place a few florets of broccoli into each pepper; cover with shredded Colby-Monterey Jack cheese.
Bake in the preheated oven until peppers are heated through and cheese is melted, about 10 minutes. Remove from oven and sprinkle each pepper with 1 teaspoon grated Parmesan cheese.