Preheat your oven to 400°F. This is to get those brioche buns perfectly golden and crisp later on.
In a large sauté pan over medium-high heat, add the ground chicken. Break it apart with a spatula and cook until it starts to brown. Stir in the diced onions and cook together until the onions are soft and the chicken is fully cooked through about 7–8 minutes.
Once the chicken and onions are ready, stir in the marinara sauce, basil pesto, salt, and black pepper. Let everything simmer together for 2–3 minutes to deepen the flavor.
Lower the heat and sprinkle in the shredded Parmesan cheese. Stir until melted and combined. Let it simmer on low while you prep the buns this helps the flavors marry and thicken slightly.
Place the brioche buns cut-side up on a baking sheet. Spread each half with garlic margarine or butter. Toast in the preheated oven for about 5 minutes, until lightly golden.
Spoon a generous amount of the hot chicken mixture onto each bottom bun. Top with a hefty sprinkle of shredded mozzarella, then crown with the top bun.
Serve immediately while the buns are crisp, the cheese is gooey, and the filling is steamy. Prepare napkins it’s gloriously messy.