Preheat your oven to 400 degrees.
Begin by placing a large skillet over medium heat. Add the oil and onion, sauteing until golden brown; about 4 - 5 minutes. Meanwhile, thoroughly drain the soaked shiitake mushrooms, reserving the liquid.
Finely dice the mushrooms and stems before adding them to the skillet, along with the cremini mushrooms and garlic.
Continue to cook, stirring frequently, until the vegetables have softened and become aromatic; about 5 minutes. Add the tempeh and stir to incorporate. Saute for 5 - 8 minutes, until lightly browned.
Sprinkle the flour into the skillet, followed by the thyme, rosemary, pepper, and salt. Cook for just 1 minute before gently pouring in the mushroom stock, red wine, and Worcestershire.
Bring to a boil and cook for 4 minutes or until thickened. Stir in peas and remove the skillet from the heat.
To prepare the mashed potatoes, place the potatoes in a large saucepan and add water to cover. Set over medium heat on the stove and bring to a boil.
Reduce the heat to medium-low to maintain a gentle simmer and cook, uncovered, until the pieces are fork-tender; 20 - 25 minutes. Thoroughly drain. Add the non-dairy milk, vegan butter, and salt, and mash thoroughly until smooth, light, and fluffy.
Spread the mashed potatoes over the mushroom mixture, using a spoon or fork to give them texture on top. Alternately, you could use a piping bag fitted with a large star tip to make neat patterns all over.
Bake for 30 minutes, until bubbling around the edges. Turn on broiler without removing the pan from the oven, and cook for 3 - 4 minutes longer, or until lightly browned. Top with parsley and serve hot.