First, make a garlicky béchamel sauce. Add butter and flour to a saucepan, then add the milk and garlic and whisk until you have a smooth sauce. Add salt and pepper to taste. Add oregano and set aside. Next make the breadcumbs.
One of the best parts of the gratin is the homemade breadcrumbs. I highly recommend making them if you can. Just take any stale bread and and it to the food processor, and process until its looks like crumbs!
Slice the vegetables pretty thinly, maybe ¼ inch slices, and then layer them in the pan(use a cast iron or any heavy bottomed pan).
I put fennel on the bottom, then broccoflower, then squash on top, then add the sauce over the top.
Last add the breadcrumbs, and place in a 180C oven for about 35-40 minutes or until the veggies are soft and the top is golden.