Place the carrots and peas in a small saucepan, cover with water and bring to a boil over high heat. Cook for 3-4 minutes until the carrots are no longer crunchy (don't overcook, they go mushy). Drain and set aside.
Preheat the oven to 400 degrees F. Grease a 9 x 13" casserole dish.
In a large saucepan, heat the olive oil over medium heat. Toss in the onion and sauté for 2 minutes until softened. Add the sliced mushrooms, tempeh crumbles, garlic and thyme and cook for about 5-8 minutes, or until the mushrooms are cooked and the onion is golden, stirring to ensure they don't stick and burn.
While that's cooking, place the cashews, soup stock, miso, and Worcestershire sauce in a blender, blending on high until smooth.
Deglaze the pan with the stout. Stir in the gravy from the blender and cook for about a minute until well combined and hot. Add in the peas and carrots, stirring well.
Pour into a 9 x 13 inch casserole dish, leveling out with a big spoon. Carefully place spoonfuls of the colcannon over the top and then smooth out with a fork or spatula, taking care not to press too hard so the stew does get pushed up the sides. Bake for 20-25 minutes until a bit crispy on top; turning on the broiler for the final few minutes adds some crispness to the potatoes and highlights any swirls you might make when spreading the potatoes. Let sit for 5 minutes and then serve.