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.