Preheat oven to 350 degrees.
Sauté onions in olive oil until translucent.
Cook broccolli, carrots and cauiflower in a small amount of water until crisp tender.
Grease a 9x13 glass baking dish with cooking spray.
Add onions, water chesnuts and steamed vegeatbles to baking dish. Sprinkle generously with season salt.
Whip cream in a bowl with an electric mixer until stiff peaks form.
Fold mayoaise and cheese into whipped cream and spread evenly on top of vegeatbels.
Bake for 45 min or until the top is brown and bubbly.
Devour.