Lentil Soup With Sausage

Name: Lentil Soup With Sausage
Ingredients: 2 tablespoon olive oil; 12 ounces kielbasa, thinly sliced (about 2½ cups); 1 ½ cup finely chopped yellow onion (from 1 onion); 2 medium stalks celery, halved lengthwise and thinly sliced (about 3/4 cup); 2 teaspoon finely chopped fresh rosemary, plus more for serving; 2 cup lower-sodium vegetable broth; 1 14.5-oz. can no-salt-added diced tomatoes; 2 14-oz. cans brown lentils, drained and rinsed; ½ teaspoon kosher salt; ¼ cup sour cream (optional)