Heat the olive oil in a large soup pot or Dutch oven over medium heat. Allow the oil to warm for about 30 seconds before adding the vegetables. Starting with warm oil helps the onions and celery soften evenly while building a flavorful base for the soup.

Add the chopped onion, celery, and minced garlic to the pot. Stir everything together and cook for about 5–7 minutes, stirring occasionally, until the vegetables become soft and fragrant. The onions should turn slightly translucent and the garlic should smell warm and savory without browning too much. Cooking this mixture slowly helps create deep flavor throughout the soup.

Stir in the sliced zucchini and continue cooking for another 3–4 minutes. The zucchini should begin softening slightly while still holding its shape. Avoid overcooking at this stage because the zucchini will continue to cook as the soup simmers.

Pour in the fire-roasted tomatoes along with the vegetable broth, then sprinkle in the chili flakes. Stir everything well so the vegetables and spices distribute evenly throughout the broth. As the soup heats up, the tomatoes and chili flakes create a rich, slightly smoky aroma that makes the soup feel warm and comforting.

Increase the heat slightly and bring the soup to a gentle simmer. Once small bubbles begin appearing around the edges of the pot, reduce the heat to medium-low and let the soup simmer uncovered for about 20 minutes. This allows the vegetables to become tender while the broth develops deeper flavor. Stir occasionally to prevent anything from sticking to the bottom of the pot.

Add the chopped kale or spinach during the final 5 minutes of cooking. Stir the greens into the hot broth until wilted and tender. Kale will stay slightly heartier and textured, while spinach softens quickly and blends more smoothly into the soup.

Turn off the heat and stir in the fresh lemon juice. The lemon brightens the soup and balances the warmth of the garlic, tomatoes, and chili flakes beautifully. Taste the broth and season with salt and black pepper as needed, adjusting the spice level if desired.

Ladle the soup into serving bowls while hot and fragrant. If you like, finish with optional toppings such as fresh parsley, cilantro, avocado slices, extra chili flakes, pumpkin seeds, or a drizzle of olive oil for added flavor and texture. Serve warm with your favorite healthy side for a cozy, nourishing meal.