Line a cookie sheet with tin foil and pre-heat the broiler in oven ((500 °F (260 °C) - or broil setting depending on oven).
Cut Roma tomatoes in half lengthwise and set on cookie sheet skin side down. Split red bell peppers in half lengthwise, remove seeds, and set on cookie sheet skin side down. Remove skin from garlic cloves and slice off the hard end, but otherwise leave whole and scatter onto cookie sheet between other vegetables.
Drizzle the veggies with the olive oil and season well with salt and pepper.
Put the vegetables into oven and roast for approximately 10 minutes until darkened and the skins are wrinkled a bit. Garlic will appear browned.
While vegetables are in the oven, section off red onion and place in a food processor.
Once vegetables are out of the oven, add to the food processor (a bit at a time) and blend fully until a slightly chunky sauce develops. Scrape the olive oil from the roasted vegetables cookie sheet into the food processor, and add the balsamic vinegar and basil (8-10 leaves of fresh) to the processor as well. Blend fully.
Serve sauce warm or cool with any number of dishes. You can also easily add other vegetables you have laying about like mushrooms, squash and carrots.
There are 80 calories in 1 serving of Roasted Vegetable Sauce.