Put potatoes in pot, and cover with water. Bring to a boil, reduce heat to medium-low, and simmer 30 minutes, or until tender. Drain. Mash with non-dairy milk and margarine until smooth. Add chives, and season with salt and pepper, if desired. Cool.
Heat 1 tablespoon of oil in skillet over medium heat. Add onions, and cook 20 minutes, or until golden.
Preheat oven to 375°F. Coat baking sheet with cooking spray. Chop portobellos, wild mushrooms and toss with olive oil and garlic. Roast 25 minutes, or until tender. Toss with herbs, and onions.
Coat baking sheet and inside of 8 ramekins or 9-inch pie dish with cooking spray. Distribute the mushroom mixture to create a layer and dot with 2 tablespoons of margarine (divided). Top with potatoes, shaping each into dome. Bake 40 minutes, or until golden.