In a very hot pan, separately cook the vegetables over a medium-high flame with a bit of the olive oil. Cook the onions and the garlic together in the same pan in a little more oil. Set all cooked vegetables aside in a large bowl.
Make the "agrodolce": Combine vinegar and sugar in a small saucepan and reduce by half over medium-high heat for 3 to 4 minutes.
While vegetables are still warm, combine with the roasted peppers, olives, and capers. Pour the "agrodolce" vinegar over the vegetable mixture and toss to coat. Add salt and pepper to taste. Add the chopped herbs and marinara sauce and stir.
Serve the caponata at room temperature.