In a large soup pot over medium-high heat, combine the lentils, veggie stock, coriander, cumin, oregano, red pepper flakes and bay leaves. Bring to a boil, reduce heat to low. Simmer, partially covered about 25 minutes, until the lentils are tender.
Add the potatoes, spinach and butternut squash. Re-cover and cook another 15-20 minutes, until the potatoes and squash are tender.
Meanwhile, in a large skillet over medium heat, heat the olive oil, add the onions and cook, stirring, until they start to soften, about 3-4 minutes. Add the garlic and cook, stirring often, for about 2 more minutes. Add the mixture to the soup, deglazing the skillet with a small splash of water, then add the mixture to the soup pot. Add the salt and pepper, taste and add more if needed. Discard the bay leaves.
Just before serving, stir the lemon juice into the soup. Serve with slices of lemon on top. Add more lemon juice if desired. Enjoy!