Cauliflower and Mushroom Gratin is an impressive and tasty side, yet very simple to make. You can pretty much make this with any combo of veggies that pleases you.
1. 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.
2. 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.
3. 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.