In a deep saute pan or Dutch oven over medium heat, melt butter with the olive oil and cook the onions and bell peppers until caramelized. Add the bratwurst to the pan and pour the beer on top. Bring to a low simmer and cover, cooking until plump and fully cooked. Season with salt and pepper. In a nonstick pan, or using a grill, brown the brats to crisp the outsides. Serve on toasted buns, topped with the pepper-onion mixture (strain this from the beer) and mustard and kraut, if desired.