Cut the cauliflower into medium pieces and place in a bowl with the onions. Set aside.
Blend the soy milk, water, cashews, salt, white and black pepper, nutmeg, garlic powder, and nutritional yeast in a blender until smooth. Pour into a saucepan and gently heat, stirring often. Meanwhile, heat the oil in a small saucepan. Add the flour to make a roux. Then whisk the heated soy milk mixture into the roux to make a light, creamy sauce. For a thicker sauce, use more roux. Pour the sauce over the vegetables and stir.
Oil a casserole dish. Pour the vegetable and sauce mixture into the pan and press lightly. Sprinkle the bread crumbs over the vegetables and cover with paprika. Cover and bake in a preheated oven at 180C/350F for 45 to 55 minutes.