Heat 2 tablespoons of oil in a heavy large pot over medium-high heat. Add the onions, carrots, garlic, and thyme and saute until the onions are translucent, about 5 minutes. Add the broth, bell peppers, potato, wine, and sugar. Bring to a simmer over high heat. Decrease the heat to medium-low. Partially cover and simmer until the potatoes are very tender, stirring occasionally, about 30 minutes. Cool the soup slightly.
Using an immersion hand blender, puree the soup in the pot until it is smooth. Alternately, working in batches, puree the soup in a regular blender, taking care while blending warm liquids. Season the soup, to taste, with salt and pepper.
Meanwhile, heat the remaining 2 tablespoons oil in a heavy large skillet over medium heat. Add the bread cubes and saute until they are crisp and golden, about 8 minutes.
Ladle the soup into bowls. Dollop a tablespoon of mascarpone in the center of each bowl and top with croutons. Sprinkle with pepper and serve.