Heat olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add diced onion, carrots, and celery, cooking for 5-7 minutes until the vegetables soften and become fragrant and translucent.
Stir in minced garlic and cook for just 1 minute until the aroma fills your kitchen. This aromatic base creates the foundation for all the flavors that follow in your meal.
Add brown lentils, vegetable broth, canned tomatoes, bay leaf, thyme, oregano, and cumin to the pot. Stir everything together thoroughly, then bring to a boil and reduce heat to low simmer.
Simmer the savory fall lentil dinner recipe for 20-25 minutes until lentils are tender but still hold their shape. Stir occasionally and check that the liquid level stays adequate throughout cooking.
Taste and adjust seasonings with salt, pepper, and balsamic vinegar for balance. Add fresh spinach and stir gently for 1-2 minutes until completely wilted into the cozy fall dinner.
Remove the bay leaf before serving in bowls or storing in containers for later enjoyment.