Place sliced onion, olive oil, salt, and pepper in a large pot over medium heat. Sauté until golden brown, stirring occasionally. Add the garlic and stir another minute. Add the vegetable broth, lemon zest, lemon juice, and roasted fennel and bring the mixture to a simmer. Simmer for 7 minutes.
Remove the pot from heat, add the spinach, spicy greens, fennel fronds, and chives. Stir to submerge in the soup, cover, and allow the greens to wilt in the heat of the broth for a few minutes.
Transfer the whole pot of ingredients to a high speed blender and blend until completely smooth. Return to the pot, taste, and add more salt and pepper to taste.