Prepare the quinoa by rinsing it thoroughly in a fine-mesh sieve under cold water until the water runs clear. This removes the natural bitter coating called saponin.
Heat olive oil in a large pot over medium heat. Add the diced onion and cook for 3-4 minutes until translucent.
Add garlic and cook for 30 seconds until fragrant, being careful not to burn it as this will create a bitter taste.
Add the carrots, celery, and bell pepper to the pot and cook for 5 minutes, stirring occasionally, until vegetables begin to soften.
Stir in the diced zucchini and canned tomatoes with their juice, cooking for another 2 minutes to incorporate flavors.
Pour in the vegetable broth and add bay leaves, thyme, oregano, and red pepper flakes if using. Bring the mixture to a boil.
Add the rinsed quinoa to the boiling soup, then reduce heat to a simmer. Cover and cook for 15-20 minutes, or until quinoa is tender and has sprouted its little “tail.”
Stir in the chopped greens (spinach or kale) and cook for 2-3 minutes more until wilted.
Remove from heat and add lemon juice, which brightens all the flavors. Season with salt and pepper to taste.
Remove bay leaves before serving. Ladle into bowls and garnish with fresh parsley.