Preheat oven to 350°F.
Place sliced zucchini in a collander and sprinkle with 2 Tbsp kosher salt. Toss and set aside for 45 minutes. Place collander on a baking sheet to catch water.
Meanwhile, in a medium sauce pan on medium heat melt 3 Tbsp butter. Add cleaned, diced leeks to melted butter and sauté until translucent, approximately 5-7 minutes.
Add flour, salt and pepper to butter and leeks, making a roux.
Turn heat down to low. Gradually whisk milk into leek roux. This will be thick, a paste like consistancy. This is fine, the zucchini will release more water while baking and will loosen the thick cream sauce.