Never tried cooking brats like this before, but wow, what a game-changer!

Preheat your oven to 350°F (175°C).
In a large oven-safe skillet or Dutch oven, heat the olive oil and butter over medium heat.
Add the sliced onions and cook, stirring occasionally, until they are soft and golden brown, about 20-25 minutes. This slow caramelization process brings out the natural sweetness of the onions.
Stir in the minced garlic and cook for another 2-3 minutes until fragrant.
Nestle the bratwursts among the onions in the skillet.
Pour the beer and beef broth into the skillet, ensuring the brats are mostly covered by the liquid. Stir in the Dijon mustard, and season with salt and pepper to taste.
Bring the mixture to a simmer on the stovetop, then transfer the skillet to the preheated oven.
Bake, uncovered, for 30-40 minutes, or until the bratwursts are cooked through and have absorbed the rich flavors of the braising liquid.

If desired, garnish with chopped fresh parsley before serving.
Variations & Tips
For a different twist, you can add sliced bell peppers along with the onions for an extra burst of color and flavor. If you prefer a spicier kick, use a beer with more hops, like an IPA, or add a pinch of red pepper flakes. For those who enjoy a touch of sweetness, try adding a spoonful of brown sugar to the onions during caramelization. And remember, if you have any leftovers, these beer brats make an excellent addition to sandwiches or can be sliced and served over pasta for another comforting meal.