Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Press the extra firm tofu between paper towels to remove excess moisture, then cut into half-inch cubes.
Dice the carrot, celery, and yellow onion into uniform pieces for even roasting.
Place the tofu and diced vegetables on the baking sheet, drizzle with olive oil, and sprinkle with sea salt and black pepper.
Toss to coat and roast for 20 minutes, or until the tofu is golden and the vegetables are tender.
While the vegetables roast, combine the dry green lentils, vegetable broth, minced garlic, and dried thyme in a large pot.
Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for 20 to 25 minutes until the lentils are soft but not mushy.
Stir the roasted tofu and vegetables into the pot along with the nutritional yeast to add a savory depth.
Add the fresh spinach and stir until the leaves are just wilted into the hot soup.
Ladle the soup into a large bowl and enjoy immediately.