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 softens.
Add the mushrooms and cook for 5 minutes, stirring occasionally, until they release their liquid and start to brown.
Stir in the carrots and celery and cook for another 3 minutes.
Add the lentils, Guinness stout, vegetable broth, thyme, and bay leaves. 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 bay leaves. Season with salt and black pepper to taste.
Ladle the stew into bowls, top with chopped parsley, and serve with crusty bread.