Boil vegetables with some salt and just enough water. Steam the chopped spinach and drain off the water from the vegetables. See that the vegetables do not overcook.
To prepare white sauce:
Melt the butter on a low heat, add flour and stir with a whisk till you get a nutty aroma.
Add milk gradually, stirring all the while with a whisk so that no lumps are formed. Simmer on low heat stirring all the while till the mixture thickens.