Heat olive oil in a large pot or Dutch oven over medium heat. Add the diced onion and sauté for 3-4 minutes until it becomes translucent.
Add the minced garlic and cook for another 30 seconds until fragrant, being careful not to burn it.
Incorporate the carrots and celery, cooking for 5 minutes until they begin to soften.
Add the sliced mushrooms and increase heat to medium-high. Cook for 7-8 minutes, stirring occasionally, until the mushrooms have released their liquid and begun to brown. This develops their deep umami flavor.
Stir in the tomato paste and cook for 1 minute to remove the raw taste and enhance the richness of the stew.
Add the lentils, diced tomatoes, vegetable broth, bay leaves, thyme, soy sauce, smoked paprika, and rosemary. Stir to combine all ingredients thoroughly.
Bring the mixture to a boil, then reduce heat to low, cover partially, and simmer for 25-30 minutes, or until the lentils are tender but not mushy.
Stir in the balsamic vinegar during the last 5 minutes of cooking to brighten the flavors.
Taste and adjust seasoning with salt and pepper as needed. Remove bay leaves before serving.
Ladle into bowls and garnish with freshly chopped parsley. Serve hot with crusty bread for a complete meal.