In large mixer bowl, combine 1 cup bread flour, yeast, onion, caraway seed and salt; mix well. Add warm water (120-130F), molasses and oil to flour mixture. Blend at low speed until moistened; beat 3 minutes at medium speed. By hand, gradually stir in rye flour to make a stiff batter. Spread in greased 1 ½ quart shallow baking dish. Combine mayonnaise and mustard; spread over dough. Cover; let rise in warm place until almost doubled, about 20 minutes.