Begin by rinsing the lentils under cold water. Set aside.
Heat the olive oil in a large pot or Dutch oven over medium heat. Add the diced onion and sauté for about 5 minutes, or until translucent.
Stir in the minced garlic, diced carrots, and diced celery, and cook for an additional 5–7 minutes until the veggies are softened.
Add the sliced mushrooms to the pot, cooking for another 5 minutes until they have released their moisture and are nicely browned.
Mix in the dried thyme, dried rosemary, and smoked paprika. Stir for about 1 minute to toast the spices.
Pour in the vegetable broth and canned diced tomatoes, followed by the rinsed lentils and the bay leaf. Stir well to combine.
Bring the stew to a boil, then reduce the heat to low and cover. Let it simmer for about 30–35 minutes, or until the lentils are tender.
Once cooked, season the stew with salt and pepper to taste. Remove the bay leaf before serving.
Serve hot, garnished with fresh parsley if desired.