Combine zucchini, squash, leek, broth, 1/2 teaspoon salt, 1/2 teaspoon pepper, and garlic in a Dutch oven. Cover and cook over medium-high heat 20 minutes or until squash is very tender, stirring occasionally. Uncover and remove from heat; cool slightly.
Combine remaining 1/2 teaspoon salt, remaining 1/2 teaspoon pepper, sour cream, milk, and egg whites in a large bowl, stirring with a whisk. Add squash mixture, basmati rice, cheeses, parsley, and oregano; stir well to combine. Pour mixture into a 13 x 9-inch baking dish coated with cooking spray.
Place crackers in a food processor; process until coarsely ground. Drizzle with butter; pulse 3 times or until moist. Sprinkle cracker crumb mixture evenly over rice mixture. Bake at 375° for 25 minutes or until browned on top and bubbly around edges. Let stand 10 minutes before serving.