Combine the onions, beer, vinegar, salt, mustard seeds, celery seed, bay leaf, and black pepper in a 4-quart saucepan, bring to a simmer, and simmer gently over medium-low heat for 20 minutes.
Add the bratwurst and simmer gently until cooked through, 8 to 10 minutes.
Remove the bratwurst and set aside. Strain the poaching liquid reserving the onions. Let cool. Remove the bay leaf, add crushed red pepper, olive oil, and chopped fresh parsley. Set aside to use as a relish.
Place the bratwurst on the grill and cook over medium-high until charred. Butter the rolls and toast on the grill just before serving.
Recipe courtesy of David Page and Barbara Shinn, from "Recipes from Home", Artisan 2001.