Heat olive oil in a large pot or Dutch oven over medium heat. Add chopped onion, carrots, and celery. Cook until softened, about 5-7 minutes.
Add minced garlic, dried thyme, and dried rosemary. Cook for 1 minute more until fragrant.
Pour in the vegetable broth, add the rinsed red lentils, diced tomatoes (with their juice), and rinsed quinoa. Bring to a boil.
Reduce heat to low, cover, and simmer for 30-40 minutes, or until lentils and quinoa are tender. Stir occasionally.
Stir in the fresh spinach and cook until wilted, about 2-3 minutes.
Season with salt and pepper to taste.
Ladle soup into bowls and serve hot.
Notes
This lentil soup is hearty and satisfying. Feel free to add other vegetables like bell peppers or zucchini. For a creamier texture, you can blend a portion of the soup before adding the spinach.