Kielbasa and Lentil Soup

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.

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

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.

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.

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

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).

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

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.