Coat the bottom of a large pot with the oil and place it over medium heat. Give the oil a minute to heat up, then add the onion, carrots, and celery. Cook the vegetables for about 10 minutes, stirring occasionally, until they begin to soften.
Stir in the garlic, cumin, and smoked paprika. Cook everything for about 1 minute longer, stirring constantly, until the mixture becomes very fragrant.
Stir in the broth and lentils. Raise the heat and bring the liquid to a boil, then lower the heat and let it simmer for 15 minutes.
Stir in the quinoa and sweet potato, then let the soup continue to simmer for about 15 minutes more, stirring occasionally, until the quinoa, lentils, and sweet potato are tender. You can add some water to the pot if it reduces too much at any point.
Stir in the diced tomatoes and let the soup simmer for 5 minutes more, stirring occasionally.
Add the spinach and simmer the soup just until the spinach is completely wilted, 1 to 2 minutes longer.
Remove the pot from heat. Season the soup with salt and pepper to taste.
Ladle into bowls and serve.