Preheat the oven to 400°F.
Place the bell peppers, tomatoes, onions, and garlic in a sauté pan or shallow baking dish and toss with the olive oil and 2½ teaspoons of the salt. Roast for about 40 minutes, or until the vegetables are just starting to brown.
Transfer the vegetables with their juices to a food processor or blender. Add the rosemary, cayenne pepper, and remaining ½ teaspoon salt and purée until smooth.
When done, transfer the mixture to a bowl and add the water and balsamic vinegar. Chill in the refrigerator until cold (at least 2 hours).
Taste and adjust seasoning if necessary, and serve cold.