Caramelized Carrots and Onions with Whole Grain Mustard

1 tablespoons vegetable oil
2 tablespoons whole-grain mustard
1½ tablespoons packed light brown sugar
½ tablespoon cider vinegar
2 medium onion, cut into 1-inch wedges
7 medium carrots, cut 1-inch thick on the diagonal
1 tablespoon butter
Salt and pepper to taste

Preparation method