For the filling, heat olive oil over medium heat and add the onions, carrots and celery; cook for 2 – 3 minutes. Add the mushrooms and cook until softened, then add the garlic and one teaspoon of course salt and cook for 5 minutes. Add the peas and cook until warmed through. Stir in the soy sauce, balsamic vinegar and herbs and cook until liquid is absorbed. Sprinkle in the flour and stir to combine, cooking for 1 – 2 minutes. Add the vegetable stock and cook until slightly thickened and nearly all liquid is absorbed. Stir in lentils and season with salt and pepper to taste.