In a large skillet over medium heat, heat olive oil. Add onion, season with salt and pepper, and cook until slightly tender, 4 minutes. Push onions to edges of skillet and add brats to center of skillet. Prick each gently with a fork to release fat. Sear brats until golden, 2 minutes per side. Add beer and simmer until all the liquid has evaporated, 10 to 12 minutes.