Preheat oven to 350°F (175°C).
Heat olive oil and butter in an oven-safe skillet or Dutch oven over medium heat.
Add sliced onions and cook until soft and golden brown, stirring occasionally (about 20-25 minutes).
Stir in minced garlic and cook for 2-3 minutes more.
Nestle bratwursts among the onions in the skillet.
Pour beer and beef broth into the skillet, ensuring brats are mostly covered.
Stir in Dijon mustard, salt, and pepper.
Bring mixture to a simmer on stovetop, then transfer to oven.
Bake uncovered for 30-40 minutes, or until brats are cooked through.
Garnish with chopped parsley (optional) before serving.
