In a large bowl, combine the flour, cheese, salt, baking powder and baking soda. In another bowl, whisk the eggs, buttermilk, sugar and butter. Stir into dry ingredients just until moistened. Fold in zucchini and onion., Pour into a greased and floured 9x5-in. loaf pan. Bake at 350° until a toothpick inserted in the center comes out clean, about 1 hour. Cool for 10 minutes before removing from pan to a wire rack.