For bock-mustard relish, in a small bowl, combine the 1/4 cup beer, vinegar, and mustard seeds. Let stand at room temperature for 3 hours. Transfer mixture to a blender or food processor. Cover and blend or process until mixture reaches a coarse consistency. Transfer to a small saucepan. Cook over low heat about 5 minutes or until mixture starts to thicken (mixture should not be as thick as yellow mustard). Cool. Stir in honey and garlic. Season to taste with salt and black pepper. Set aside.
Prick each bratwurst several times with a fork. In a Dutch oven, combine bratwursts and two bottles of beer. Bring to boiling; reduce heat. Simmer, covered, about 20 minutes or until bratwursts are no longer pink and juices run clear (160 degrees F); drain.
For a charcoal grill, grill sweet pepper and onion on the rack of an uncovered grill directly over medium coals for 8 to 10 minutes or until tender, turning occasionally to brown evenly. While vegetables grill, add bratwursts to grill. Grill for 5 to 7 minutes or until bratwursts are browned, turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place sweet pepper and onion, then bratwursts on grill rack over heat. Cover and grill as above.)
When cool enough to handle, chop sweet pepper and onion. For bock-mustard relish, stir sweet pepper, onion, pickle, and horseradish into honey mixture. Serve bratwursts in toasted buns with bock-mustard relish.