Heat olive oil in a large skillet or Dutch oven over medium heat. Add the sliced onions and cook until softened, about 5-7 minutes.
Add the bratwurst to the skillet with the onions.
Pour the beer over the brats and onions. Add salt and pepper.
Bring the liquid to a simmer, then reduce heat to low, cover, and cook for 20-30 minutes, or until the brats are cooked through. Avoid boiling, which can cause casings to split.
Remove the brats from the skillet. If desired, you can finish them on a preheated grill or in a clean skillet over medium-high heat for a crispy exterior. Sear for 2-3 minutes per side.
Lager beer will give a lighter flavor, while ale will provide a richer taste.
Simmering gently prevents the bratwurst casings from splitting.
Onions should be tender but not mushy; adjust simmering time as needed.
These beer brats are a great make-ahead option for game day. Store cooked brats and onions in their liquid in the refrigerator for up to 3 days. Reheat gently on the stovetop or in the oven.