Preheat oven to 200°C (400°F) and set out a deep aluminum foil pan or baking dish.
Heat olive oil and butter in a large skillet over medium heat, add sliced yellow onions, and cook for 10–12 minutes, stirring occasionally until soft and golden.
Add minced garlic, brown sugar, Worcestershire sauce, and Dijon mustard to the onions, stirring for 2 minutes to caramelize.
Pour in the beer, season with sea salt and black pepper, and bring to a gentle simmer for 3 minutes.
Arrange bratwurst sausages side-by-side in a single layer inside the aluminum foil pan.
Pour the hot beer and caramelized onion sauce evenly over the sausages, making sure they are well coated.
Bake at 200°C (400°F) for 20–25 minutes, flipping sausages halfway through, until juicy, cooked through, and deeply browned on top.
Serve immediately using tongs, spooning the sweet caramelized onions and rich beer glaze generously over each sausage.
