Put the potatoes in a large pot, cover with water, and simmer 25 minutes or until tender all the way through (pierce with a fork to check). When they’re done, dip out 1/2 cup of the cooking water and drain.
While the potatoes are cooking, rehydrate the porcini mushrooms according to package directions. (I cover them with hot water and soak for about 25 minutes.) When they are completely rehydrated, drain them, reserving the liquid, and chop coarsely. Strain the soaking liquid through a coffee filter or fine mesh strainer and set aside.
Heat a non-stick skillet. Add the onions and sauté until they are browned. Sprinkle them with salt and pepper to taste, then remove them from the pan.
Add the minced garlic and panko to the hot pan. Cook, stirring constantly, for about 3 minutes or until the panko has browned slightly. Sprinkle with salt and pepper to taste and set aside.
Preheat the oven to 400F.
Mash the potatoes in their skins until slightly lumpy; then stir in the soymilk, nutritional yeast, roasted red pepper, diced porcini mushrooms, and half of the peas. If the potatoes seem dry, add some of the mushroom liquid and/or potato water. You want them to be fairly moist. Season to taste with salt and freshly ground black pepper.
Spray an 8-inch casserole dish lightly with olive oil or use a silicone baking dish. Spread half of the potato mixture into the dish, followed by the remaining peas and the onions. (Add "cheese" sauce now, if you'd like.) Top with the rest of the potatoes, smoothing the top.
Bake for about 25 minutes, or until hot throughout. Top with the bread crumbs, pressing them into the potatoes slightly. Bake another 10 minutes and serve.