Step 1
Preheat the oven to 400º F.
Peel and dice the potatoes. Place in a saucepan with enough water to cover. Bring to a simmer, then cover and simmer until tender, about 20 minutes. Drain and transfer to a small mixing bowl.
Step 2
Stir the buttery spread into the potatoes until melted, then add the nondairy milk and mash until fluffy. Set aside until needed. Meanwhile, as the potatoes are cooking, heat the oil in a skillet. Add the onion and carrots and sauté over medium heat until the onion is golden. Add the cauliflower or broccoli and the mushrooms along with 1/4 cup water. Cover and cook until the vegetables are just tender.
Step 3
Add the peas, corn kernels, parsley, seasoning blend, and optional dill. Cook until the mixture is well heated through, then stir in 1/2 cup of the mashed potatoes and the optional nutritional yeast. Season with salt and pepper.
Step 4
Lightly oil a shallow 2-quart casserole dish. Pour in the vegetable mixture and pat in evenly. Spread the mashed potatoes over the top and pat down lightly. Bake for 30 to 35 minutes, or until the potatoes begin to turn golden and slightly crusty.
Let stand for 5 minutes, then cut into squares or wedges to serve.