Grilled Bratwurst with Onions Braised in Beer and Mustard

2 tbsp. bacon fat (lard or olive oil)
3 c. onion, thinly sliced
1/8 tsp sugar
12 oz. beer (dark or amber)
2 bay leaves
2 tbsp. mustard (coarse-grained)
16 oz. bratwurst (fresh, cooked or smoked varieties)
4 hoagie buns (or Italian rolls, split)

Heat fat in 12-inch deep skillet over medium heat. Add onions...