Make a lightly colored rue with the flour and oil. Add the soy milk and stir frequently while bringing to a light simmer. Simmer until it is significantly thick. Add the vegetable broth and simmer some more.
Blanch the broccoli and slice the potatoes.
Put the broccoli and potatoes into a casserole pan, pour in the thickened soy milk. Bake covered at 350 until the potatoes are done. Remove the cover and bake for a little longer to crisp up the top. It will need to sit for a little after it comes out of the oven.