Make the bechamel sauce as described here. Heat the olive oil in a large wide skillet over a medium high flame. Add the shallots and thyme and sauté for a minute. Add the leeks and continue cooking until they start to soften and begin to brown.
Add the mushrooms and a pinch of sea salt, cook, stirring until they start to darken and wilt, about 5 to 8 minutes. Add the stock or water and cauliflower, cover, and turn the heat down to low. Cook for 5 minutes.
Tip the vegetables into a baking dish and pour the Béchamel sauce over them. Sprinkle the cheeses and breadcrumbs over the top, and cook under a broiler until the top is browned and crispy, about 5 to 10 minutes.