Heat olive oil in a large pot over medium heat for one minute. Add diced onion, carrots, and celery, sautéing for 5-6 minutes until vegetables soften and release their aromatic flavors into the pot.

Stir in minced garlic and cook for exactly 1 minute to prevent burning while building the flavor base. You'll notice the kitchen smells incredible at this point, which means you're on the right track.

Pour in dried brown lentils and stir to coat with the oil and aromatics for about 30 seconds. This light toasting enhances the nutty lentil flavor before adding any liquid to the pot.

Add vegetable broth, canned diced tomatoes with their juice, bay leaf, thyme, oregano, and cumin all at once. Stir well and bring the mixture to a boil, then reduce heat to medium-low and simmer uncovered for 25-28 minutes until lentils are completely tender.

This lentil soup is ready when you can easily mash a lentil against the pot side with your spoon. If the soup seems too thick, add more broth in quarter-cup increments until you reach your preferred consistency.

Stir in fresh spinach and balsamic vinegar, cooking for just 2 minutes until spinach is completely wilted. Season generously with salt and pepper to taste, adjusting the seasoning before serving.