Heat the olive oil in a large pot or Dutch oven over medium heat.
Add the diced onion, carrots, and celery. Sauté for about 5-7 minutes until the vegetables begin to soften.
Stir in the minced garlic and sliced mushrooms. Cook for another 5 minutes until the mushrooms are tender and have released their moisture.
Add the rinsed lentils, vegetable broth, diced tomatoes (with their juice), dried thyme, oregano, and bay leaf. Bring to a boil.
Once boiling, reduce the heat to low and cover the pot. Simmer for about 25-30 minutes, or until the lentils are tender.
If using, stir in the chopped kale or spinach during the last 5 minutes of cooking.
Season with salt and pepper to taste. Remove the bay leaf before serving.
Ladle the stew into bowls and garnish with fresh parsley, if desired.