Preheat the grill to medium-high.

In a large bowl, toss the tomatoes and onions with 3 tablespoons of the oil to lightly coat. Season lightly with salt and pepper. Place the tomatoes, cut sides down, and the onions and bell pepper on the grill and cook, turning, until charred and soft, about 4 minutes for the tomatoes and 6 to 10 for the onions and pepper. Remove and place the pepper in a plastic or paper bag, and cool for 10 minutes. Let the tomatoes and onions sit until cool enough to handle; peel the tomatoes and chop, and chop the onions.  Peel and chop the bell pepper.

Place the pancetta or bacon on the grill and cook, turning, until charred around the edges and cooked through. Set aside. If using bacon, crumble into pieces.

In a large pot, heat the remaining tablespoon oil over medium-high heat. Add the garlic and cook, stirring, until soft and fragrant, about 45 seconds. Add the chopped onion, bell pepper, and tomato and cook, stirring, for 2 minutes. Add the stock, vinegar and sugar and bring to a boil. Reduce the heat to a simmer and cook, stirring occasionally, for 30 minutes. Remove from the heat and puree with a hand-held blender. (Alternatively, transfer in batches to a food processor and puree, and return to the pot.) Add the cream, if using, and basil, return to a simmer, and cook, until warmed through, 2 to 3 minutes. Adjust the seasoning to taste.

Ladle the soup into 4 shallow soup bowls and top each with 1 grilled pancetta round or crumbled bacon. Serve immediately.