Prepare the seasonal vegetables as appropriate and cut into largish pieces. Peel and roughly chop the onions, peel and crush the garlic. Grate the cheese for the topping.
Put the root vegetables, including the onion and garlic into a roasting tray and mix with the oil. Roast in the preheated oven for 15 minutes. If using vegetables such as broccoli or cauliflower, blanch in boiling water for a few minutes.
For the topping, put the flour into a large mixing bowl. Cut the margarine into small cubes and rub into the flour. When it looks like breadcrumbs, add the oats and cheese and mix in. Put to one side.
To make the sauce, melt the butter or margarine in a pan, then add the flour and cook over a gentle heat until the mixture turns sandy in colour and texture. Gradually add the milk, beating all the time, and cook until the mixture thickens. Continue to cook for a further 5 minutes over a low heat.
Put the vegetables on the bottom of a large square, deep dish then add the sauce. Finally sprinkle the topping over the dish. Cover with foil and bake in the preheated oven for about 15 minutes. Uncover and bake for another 15-20 minutes or until the cheese topping is bubbling and golden.