Roux method for white sauce:
1oz flour Plain or SR
1oz butter
1/2 - to a pint of milk.
Salt and pepper
Melt butter over a medium heat, when melted stir in the flour and cook the 'paste' for a few mins stirring all the time. gradually add the milk using a whisk so that there are no lumps make sure you scrape around the sides and bottom of pan so nothing sticks, when it's as thick/thin as you want, add cheese and stir until melted. It's important to keep stirring all the time so you get no lumps.
For the pasta bake you can use any veg you want, carrots, sweetcorn, just make sure that your mixture is nearly cooked as it'll finish cooking in the sauce