German Sausages with Peppers and Onions

Ingredients

• tablespoons butter or canola oil
• large German sausages, such as bratwurst
• large onion, halved and cut into ¼-inch slices
• green bell pepper, seeded and cut into ¼-inch rings
• red bell pepper, seeded and cut into ¼-inch rings
• bottle German-style lager
• Kosher salt
• Freshly ground black pepper
• hoagie rolls
• Good-quality mustard, for serving