Begin by pricking the brats all over with a fork. Using a medium pot add the bratwurst, beer, and half the onions. Bring this to a simmer over a medium high heat for 15 minutes or until brats are firm and cooked through. Place brats on a plate and save some of the cooking liquid.
In a large skillet heat oil over medium high heat. Add the rest of the onion and bell peppers and cook for about 15 minutes or until vegetables are soft, tossing pan occasionally. During the last 5 minutes of cooking add brats and lightly brown. If the vegetables are getting to dark add a few tablespoons of the bratwurst cooking liquid. Season to taste with salt and pepper.
To serve place brat in bun spread with mustard and top with sauteed peppers and onions.