In a large pot, boil the potatoes until almost done. When slightly cooled, peel off the skin and slice into 1/4 thick coins. Put aside in a large baking/casserole dish. Steam the broccoli and cauliflower and mix with the potatoes in the dish.
Warm the oil and sauté the onions and garlic until soft and transparent. slowly, while stirring, add the flour. It should become very thick, like a dough, in the pan. Whisk in the soymilk to thin it out, and bring to a boil slowly on medium heat while stirring. Pour into a blender and add the silken tofu, pepper, salt, and 5 tablespoons of the parmesan. Blend until smooth and creamy.
Pour creamy mixture over the potatoes and vegetables, top with remaining parmesan, bread crumbs, and some more black pepper, and bake in a 250C oven for one hour until brown and bubbly.