Position a rack in the center of the oven and place a 10-inch castiron skillet on it; preheat the oven to 450 F.
Whisk together the cornmeal, pastry flour, baking powder, baking soda, salt, oregano, pepper, and pepper flakes in a large bowl. Make a well in the center. In a medium bowl, lightly whisk the eggs to blend. Gently whisk in the oil, buttermilk, and thyme until smooth. Add the egg mixture to the center of the flour mixture, and stir with a rubber spatula until just combined. Do not overmix; the batter should look lumpy. Fold in the feta cheese.
Using a thick oven mitt, carefully remove the hot skillet from the oven. Add the butter and tilt carefully to coat the bottom and the sides. It will sizzle and brown fast for great flavor. Scrape the batter into the hot skillet, leveling the top with a spatula. Reduce the oven temperature to 400°F.
Bake until the edges of the cornbread turn golden brown and a toothpick inserted into the center comes out clean, 20 to 25 minutes. Let sit for 5 minutes before cutting into wedges. Serve
TO VARY IT: For a deliciously textured, more rustic cornbread, replace 1 cup of the medium-coarse cornmeal with coarse stoneground cornmeal.