1. Cut each bratwurst crosswise into 2 to 3 pieces. Cut each onion into 8 wedges. In 3-quart casserole or bowl, combine bratwurst and onions; cover with casserole lid or large plate. Cook at high (100% power) 12 minutes, stirring mixture every 4 minutes and pushing cooked pieces to center. Drain well; return bratwurst and onions to casserole.
2. In 2-cup glass measuring cup, stir beer, flour, mustard, salt, and pepper until smooth; pour mixture over drained bratwurst mixture.
3. Pierce each potato skin with fork in several places and arrange on top of bratwurst mixture around edge of casserole; cover with casserole lid or large plate. Cook at high 10 minutes, stirring after 5 minutes.
4. Separate frozen green beans under running warm water and drain on paper towels. Stir beans into bratwurst mixture. Cover and cook at medium high (70% power) 20 to 25 minutes until potatoes and beans are tender, stirring every 5 minutes.