Whisk together brown sugar, mustard and molasses in medium-size bowl until smooth.
Place 1/2 cup beans in a food processor or blender. Whirl until pureed. Stir into brown sugar mixture along with remaining beans, green and red peppers and onion. Pour beans into 2-quart baking dish.
Bake, uncovered, in preheated 350 degree F oven for 1 hour 20 minutes until slightly thickened. Makes 8 servings.