Rinse the dry lentils thoroughly under cold water and set aside.
Press the tofu to remove excess moisture, then crumble it into small, bite-sized pieces.
In a large pot, heat the olive oil over medium heat. Add the onion, carrots, and celery, sautéing until the onion is translucent.
Stir in the minced garlic and crumbled tofu, cooking for another 2 minutes until the garlic is fragrant.
Add the lentils, vegetable broth, dried thyme, sea salt, and black pepper to the pot.
Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for 20-25 minutes or until the lentils are tender.
Stir in the fresh spinach and nutritional yeast, allowing the spinach to wilt for about 1 minute.
Ladle the stew into a bowl and top with a generous dollop of non-fat Greek yogurt before serving.