Heat olive oil in a large pot or Dutch oven over medium heat.
Add the diced onion, carrots, and celery; sauté for 6 to 8 minutes until the vegetables begin to soften.
Stir in the minced garlic, dried thyme, and ground cumin, cooking for 1 minute until fragrant.
Add the dry lentils, crushed tomatoes, and vegetable broth to the pot.
Bring the mixture to a boil, then reduce heat to low and simmer, partially covered, for 30 to 35 minutes until the lentils are tender but not mushy.
Stir in the chopped spinach and cook for 2 minutes until wilted.
Remove from heat, stir in the lemon juice, and season with salt and black pepper before serving.