Heat olive oil in a large pot over medium-high heat. Add onion, garlic, carrots, and celery. Sauté for 5 minutes until the vegetables soften and the onion is translucent.

Stir in diced tomatoes, brown lentils, and vegetable broth. Add cumin, coriander, and kosher salt. Mix thoroughly.

Bring to a boil, then reduce heat and simmer for 25 minutes, stirring occasionally, until lentils are tender.

Add baby spinach and lemon juice to the pot. Stir gently until the spinach wilts.

Adjust seasoning if needed, serve hot in bowls, optionally garnishing with herbs and enjoy.