Drain and rinse the onions, set aside.
Spray a shallow baking dish with non stick vegetable oil.
Heat in a small saucepan the mustard, sugar and cayenne for a few minutes.
Mix sauce and the drained onions.
Pour into the dish and sprinkle cracker crumbs evenly over the top.
Preheat oven to 350 degrees.
Bake about 30 minutes.