Lentil Soup with Sausage and Mustard Greens

Name: Lentil Soup with Sausage and Mustard Greens
Ingredients: 2 tablespoons olive oil, plus more for serving; 3 medium carrots, peeled, chopped; 1 medium onion, chopped; ½ fennel bulb, chopped, plus fennel fronds for serving; 3 garlic cloves, finely chopped; Kosher salt, freshly ground pepper; 1 pound sweet Italian sausage, casings removed; 2 bay leaves; 1 cup lentils (such as black beluga, French, brown, or green); ½ teaspoon crushed red pepper flakes (optional); 8 cups chicken stock or low-sodium chicken broth; 1 bunch mustard greens, torn into bite-size pieces; 1 teaspoon red wine vinegar; Sour cream and crusty bread (for serving)