Place the mustard seeds and caraway seeds in a small resealable plastic bag and crush them with a meat mallet or rolling pin. Melt the butter in a Dutch oven or a large, wide, heavy-bottomed pot over medium heat until foaming. Add 6 of the bratwursts and cook, turning occasionally, until browned on all sides, about 10 minutes total. Remove to a plate and repeat with the remaining 6 bratwursts. Reduce the heat to low, add the crushed seed mixture, onions, and brown sugar, and season generously with salt and pepper. Cook, stirring occasionally, until the onions have softened and are beginning to brown, about 40 minutes. Add the reserved bratwursts and any accumulated juices, nestling the brats in the onions. Pour in the beer and braise the bratwursts, turning once, until they are completely cooked through, about 30 minutes. Place a bratwurst in each bun and serve with the onions and mustard on the side.