In a skillet, over medium heat, cook the onions, stirring frequently, until browned. About 5 min.
Add a splash of veggie broth or water to the pan so the onions don't burn. This also helps to release the flavor.
Add the garlic and mushrooms and continue to cook until the mushrooms have softened. Add more veggie broth to keep the mixture from burning. About 5 min.
Add 1 cup of vegetable broth to the mixture.
To the remaining cup of broth, stir in the Miso, flour and tamari until dissolved. Add to the pan and stir until the mixture thickens into a gravy. About 3-5 minutes.
Add carrots, celery, peas, broccoli, thyme, rosemary, and sage.
Season with black pepper to taste.
Using a 9 x 13 inch baking dish or cast iron pan, spread the filling on the bottom, then top with the mashed potatoes mixture.
Bake 30 minutes until the top of the potatoes brown.