Heat grill to medium heat.
Cook and stir onions in hot oil in large skillet on medium-high heat 5 min. or until onions are golden brown; cover. Simmer on low heat 10 min., stirring occasionally.
Meanwhile, bring beer to boil in medium saucepan. Add bratwurst; cook 5 min. Drain. Grill 4 to 5 min. or until done (160ºF), turning occasionally.
Spread cut sides of rolls with mustard; fill with bratwurst, Singles and onions.