Combine the dry ingredients, then cut the butter into the dry mixture until it resembles coarse crumbs. Add the cheddar, onion, and pimento, mixing well. Add the milk, mixing until just moistened. Spread the dough in a 9-inch square baking pan and bake at 450 degrees F. for 25 to 30 minutes or until a wooden pick inserted in the center comes out clean. Cool slightly and cut into squares. Serve warm.