Oven Braised Beer Brats with Caramelized Onions and Apples

2 tablespoons olive oil
2 tablespoons butter
2 large onions, thinly sliced
2 apples, cored and sliced
3 cloves garlic, minced
6 bratwursts
1 cup beer
1 cup beef broth
2 tablespoons Dijon mustard
Salt and pepper to taste
Fresh parsley for garnish (optional)