Cut the feta in ¼-inch-thick slices, then cut each rectangular slice in half crosswise to form two squares. You should end up with about 16 squares. Transfer the squares to a paper towel on your cutting board and pat dry.

In a large shallow bowl or rimmed plate, stir together the cornstarch and sesame seeds. Working with one at a time, gently coat the feta on both sides with the cornstarch mixture.

In a large, nonstick skillet, melt the butter over medium heat. Add the feta in a single layer and cook until golden-brown, about 1 minute. 

Use a nonstick-safe spatula to flip and cook until golden-brown on the other side. Transfer to a platter and sprinkle with flaky salt, black pepper and red-pepper flakes. 

Serve warm with crackers or toasted baguette slices, with lemon wedges alongside for squeezing on top.