Heat oil in a large pot over medium heat.
Add onion, carrot, and celery along with a couple pinches of salt and pepper. Cook for about 8 minutes until tender.
Add mushrooms, garlic, thyme, oregano, sage, and more salt and pepper. Cook for another 3–4 minutes until mushrooms are tender.
Sprinkle in flour and stir well to coat everything evenly. Cook for an additional minute.
Pour in balsamic vinegar and soy sauce to deglaze the pan.
Add potatoes, lentils, tomato sauce, broth, bay leaves, plus more salt and pepper. Stir until well combined.
Bring the mixture to a simmer over medium-high heat while stirring occasionally.
Lower the heat and let it simmer gently for about 10–15 minutes or until potatoes are fork-tender.
Garnish with fresh parsley if desired and serve.