Preheat oven to 425ºF and position a rack in the middle. Line a 13x18-inch baking sheet with parchment paper.
In a large bowl, whisk together flour, cornstarch, baking powder, baking soda, salt, sugar, granulated onion, and granulated garlic.
Add the butter chunks. Using a pastry cutter or two knives, cut the butter into the dry ingredients until it looks like coarse sand with some pea-sized butter clumps. Alternatively, you can rub the butter into the mixture with your fingertips.
Mix in the shredded cheese. Add buttermilk and stir with a rubber spatula until the dough forms a shaggy, sticky mass. If dry, add 1-2 tablespoons more buttermilk. Avoid over-mixing.
Use a large ice cream scoop to drop the biscuits onto the prepared baking sheet. Space them evenly and do not compact or smooth them; they should be irregular in shape.
Bake for 17-20 minutes, until biscuits are lightly golden.
In a small bowl, mix melted butter, granulated onion, granulated garlic, and dried parsley. Brush this mixture over the tops of the warm biscuits.
Enjoy the biscuits warm.