while your oven is heating get out at large sauce pan . turn your heat on med-high . add your unsalted butter once your butter is melted add your flour. once your flour and butter are combine add your warm milk stir with a wooden spoon . add your nutmeg your ground mustard and both your black and white pepper to your white sauce . once your sauce has thicken add your 2cups of the white sharp cheddar cheese and your 1 cup of mild cheddar cheese once your cheeses have melted into your white sauce turn off the heat and set aside .
in a casserole dish i use a 9 by 13in casserole dish ( make sure you lightly spray your dish with cooking spray) add your thinly sliced squash zucchini and onions and sprinkle them with the 3 pinches of sea salt . pour your cheese sauce over the veggies and top with your 1/2 cup of yellow sharp cheddar cheese . bake for 25 mins or until the cheese is melted and the veggies are tender let it stand for about 15 mins .. enjoy :)