Peel 4 onions (8 oz. each) and sliver lengthwise. Rinse, stem, and seed 2 red bell peppers (1 lb. total); sliver lengthwise.
In a 6-quart pan over medium-high heat, mix 3 tablespoons olive oil and 2 teaspoons caraway seeds with onions, bell peppers, and 3 tablespoons water. Cover and cook, stirring occasionally, until vegetables are limp, 12 to 15 minutes.
Uncover and stir often over medium heat until liquid has evaporated and vegetables begin to brown and taste sweet, 20 to 25 minutes. Stir in 1 tablespoon balsamic vinegar and salt and pepper to taste.
Serve with toasted baguette slices.