Into a 2 quart microwave safe casserole place the bratwursts, sauerkraut, and beer. If you want to add the onion, peel and slice up the onion and add the onion slices to the bowl before you add the beer.
Cover the bowl and microwave at high for 25 minutes. Remove the bowl from the microwave. Remove the brauts from the bowl. Place bowl back in the microwave. Heat on high for 25 minutes.
Now grill the brauts on you backyard grill or on your stove top grill. Plate and serve when the sauerkraut is ready. Your guests may use the rolls or the plate.