Kielbasa and Lentil Soup with Lots of Basil

• 2 medium onions
• 2 celery stalks
• 1 fennel bulb
• 6 garlic cloves
• 12 oz. kielbasa
• ¼ cup extra-virgin olive oil, plus more for drizzling
• ½ tsp. crushed red pepper flakes

Kosher salt

• 2 Tbsp. tomato paste
• 1¼ cups lentils
• 2 cups basil leaves
• ½ cup whole-milk Greek yogurt

1. Prep the mirepoix (a.k.a. the aromatic base of your soup). Peel 2 medium onions and finely chop. Transfer to a large bowl. If there are any pale, tender leaves attached to the heart of your bunch of celery, pluck off about ¼ cup to reserve for serving. Take 2 celery stalks and cut in half lengthwise, then cut crosswise into ½" pieces; add to bowl with onion. Cut off stems and dark green parts from 1 fennel bulb; discard or save for another use. Cut bulb in half through the core, then finely chop. Add to bowl with onion. Lightly smash 6 garlic cloves to release skins. Peel, then smash forcefully with the side of your knife to break them down into smaller bits. Add to bowl with onion.
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3. Slice 12 oz. kielbasa crosswise into ¼"-thick coins.
4. Heat a large heavy pot over medium. Add ¼ cup extra-virgin olive oil and swirl to coat. Cook kielbasa in a single layer (it’s okay if it’s not a perfect one), undisturbed, until golden brown underneath, about 3 minutes. Turn pieces with a wooden spoon and cook, stirring occasionally, until mostly browned on both sides, 1–2 minutes longer.
5. Reduce heat to medium-low. Add onion mixture, ½ tsp. crushed red pepper flakes, and 1 tsp. salt and cook, stirring often, until vegetables are softened and onion is translucent, 8–10 minutes.
6. Add 2 Tbsp. tomato paste and cook, stirring often, until it darkens in color, 1–2 minutes longer.
7. Add 1¼ cups lentils, 1 tsp. salt, and 4½ cups water and stir to combine. Increase heat to medium-high and bring to a simmer. Once simmering, reduce heat to medium-low, cover pot, and cook, peeking occasionally to ensure soup is at a bare simmer, until lentils are tender but not falling apart, 20–45 minutes (depending on what kind of lentils you're working with).
8. Remove from heat. Add 2 cups basil, stirring to wilt. Taste and adjust seasoning for salt.
9. Divide soup between bowls. Dollop with ½ cup whole-milk Greek yogurt and drizzle with more olive oil. Garnish with reserved celery leaves if you have them.