1 tbsp olive oil
2 leeks, white and light green parts only, thinly sliced (3 cups) and rinsed in cold water
4 shallots, thinly sliced (1 1/2 cups)
1 each white and red onion, thinly sliced (3 cups each)
2 tbsp white wine vinegar
8 cups low-sodium chicken or vegetable broth
3 sprigs fresh thyme
2 bay leaves
4 slices whole-grain baguette, about 4 inches long and 1/4-inch thick each
2 oz goat cheese, softened
2 scallions, thinly sliced
Sea salt and fresh ground black pepper, to taste
1/4 cup finely sliced chives
In a large 6-qt saucepan, heat oil on medium-high. Add leeks, shallots and white and red onions. Cook, stirring frequently, for 2 to 3 minutes, until onions have softened slightly. Reduce heat to medium, cover with tight-fitting lid and allow onions to sweat and release water, about 15 to 20 minutes, stirring twice.
Remove lid and continue cooking for about 10 minutes. Reduce heat to medium-low and cook for an additional 20 minutes, stirring and scraping bottom of pan with wooden spoon every 10 minutes, allowing onions to caramelize and turn golden brown. (NOTE: Onions will be very soft and appear almost puréed.)
Increase heat to high, add vinegar and stir until liquid is incorporated. Add broth, thyme and bay leaves and bring mixture to a boil. Reduce heat to medium and simmer for about 30 minutes.
While soup is simmering, prepare crostini: Position oven rack in top third of oven and preheat to broil. Place baguette slices on a baking tray and toast in oven for about 30 seconds to 1 minute per side, until very golden brown. Set aside. In a small bowl, stir together goat cheese and scallions. Season with salt and pepper. Spread about 1 1/2 tsp goat cheese mixture onto each baguette slice and broil for about 1 minutes, until cheese is hot and light golden brown.
Remove bay leaves and thyme from soup. Sprinkle with chives and season with salt and pepper. Ladle soup into bowls, float 1 crostini on top of each bowl and serve immediately.