Step 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.

Step 2

Slice 12 oz. kielbasa crosswise into ¼"-thick coins.

Step 3

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.

Step 4

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.

Step 5

Add 2 Tbsp. tomato paste and cook, stirring often, until it darkens in color, 1–2 minutes longer.

Step 6

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).

Step 7

Remove from heat. Add 2 cups basil, stirring to wilt. Taste and adjust seasoning for salt.

Step 8

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.