Combine all the vegetables and steam just until tender. In a large saucepan, combine the steamed veggies, 2 tablespoons of margarine, flour, vegetable broth, soy milk and sage. Cook until the sauce begins to thicken and add a bit of salt and pepper, to taste. Set aside.
Boil the potatoes until soft. Drain and mash with 2 tablespoons margarine, yogurt and cheese.
Spread the vegetables in a large baking dish, and spread the potato mixture on top. Sprinkle additional parmesan cheese on top if desired.
Bake for 30 to 40 minutes or until lightly golden.