Heat a large nonstick pot over medium-high heat; when it's warm, add ¼ cup of the broth, the bell peppers, and onions. Sauté for 5 to 6 minutes, stirring occasionally.
Let the peppers and onions sit in the pot for an additional 60 seconds after all the liquid evaporates from the pan. Deglaze the pan by pouring in an additional ¼ cup broth, then add the thyme, black pepper, and diced tomatoes. Sauté for an additional 1 to 2 minutes, then add the lentils, quinoa, the remaining vegetable broth, and 2 cups water.
Bring the soup to a boil over high heat, then cover and reduce the heat to medium-low. Simmer for 23-25 minutes, until the lentils are tender. Remove from the heat and stir in the spinach until just wilted, then ladle the soup into serving bowls and serve warm.
Notes
For the salt, I would say season it to your liking. I personally use a vegetable bouillon concentrate which has plenty of salt in it so I only needed a few additional pinches. Give the soup a taste as you go and season as needed.I garnished this soup with another recipe from the cookbook, 'Baby Bella Bites'. These are halved mushrooms that are tossed with tamari, black pepper, and a sprinkling of garlic powder. They are then roasted (or air fried) until crispy!That said, there are many amazing toppings that could be added to this stew like ripe avocado or a drizzle of good cold-pressed olive oil. And of course, a big piece of toasted sourdough would be lovely!