1. In a large soup pot 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.
2. Add the lentils, stock or water, jalapeño, sumac, cumin, oregano and bay leaves. Bring to a boil, then reduce the heat to a simmer.
3. Add the potatoes, spinach, butternut squash, salt and pepper, cover and cook until the potatoes and squash are tender, another 15 to 20 minutes.
4. 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.