Heat the olive oil in a large pot over medium heat. Add the onion and garlic and cook for 3 to 4 minutes until the onion turns soft.
Add the sliced mushrooms and cook for another 5 minutes until they begin to brown and release their liquid.
Stir in the rinsed lentils, crushed tomatoes, vegetable broth, and rosemary sprigs. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 30 to 35 minutes until the lentils are tender.
Remove the rosemary sprigs, then season the stew with salt and black pepper before serving.