In a large soup pot over medium-high heat, combine the lentils, stock or water, jalapeño, oregano and bay leaves. Add coriander, cumin to a sachet bag (remove bag at the end). 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 bit of the broth from the soup, then add the deglazed 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.