Heat olive oil in a large pot over medium heat.
Add diced onion and sauté until translucent, about 5 minutes.
Add minced garlic and tomato paste, cook for another 2 minutes.
Stir in chopped carrots and celery, cook for 3-4 minutes to soften.
Add brown lentils, stock, thyme, rosemary, bay leaf, and salt.
Bring soup to a boil, then reduce heat and let simmer uncovered for 20 minutes until lentils are tender.
Remove thyme sprigs, rosemary, and bay leaf.
Stir in lemon juice and adjust seasoning to taste.
Serve hot and enjoy your hearty lentil soup.