1 pound lentils, rinsed and picked over
10 cups vegetable broth or water
1 jalapeño pepper, stemmed, seeded and chopped
2 teaspoons whole coriander seeds
1 1/2 teaspoons cumin seeds
2 1/2 teaspoons dried oregano
2 bay leaves
2 medium potatoes (1 1/4 pounds), scrubbed and cut into 1/2-inch dice
10 ounces baby spinach, chopped
1 small butternut squash (1 pound), peeled, seeded and cut into 1/2-inch dice (about 3 cups)
2 tablespoons olive oil
1 large onion, chopped
2 ribs celery, with leaves, sliced
3 large garlic cloves, finely chopped
2 teaspoons kosher salt, or more to taste
1/2 teaspoon freshly ground black pepper, or more to taste
1/3 cup fresh lemon juice
In a large soup pot over medium-high heat, combine the lentils, stock or water, jalapeño, coriander, cumin, oregano and bay leaves. Bring to a boil, then reduce the heat to low. Simmer, partially covered, about 30 minutes, until the lentils are tender.
Add the potatoes, spinach and butternut squash, re-cover and cook another 15 to 20 minutes, until the potatoes and squash are tender.
Meanwhile, in a large skillet over medium heat, heat the olive oil until shimmering. Add the onion, and cook, stirring, until it starts to soften, 3 to 4 minutes. Add the celery and garlic and cook, stirring often, until they soften, 3 minutes. Add the mixture to the soup, deglazing the skillet with a little soup liquid and adding the deglaze contents back to the soup pot. Add the salt and pepper, taste, and add more if needed. Pick out and discard the bay leaves.
Thinly slice one of the lemons and cut the other into wedges. Just before serving, stir the lemon juice into the soup. Serve the soup hot, with a lemon slice floating atop each bowl. Pass lemon wedges at the table.