Fill an 8-quart stockpot two-thirds full of water and bring it to a boil over high heat. Transfer the crabs to the pot one by one, picking up each with tongs (approach them from the back of their carapaces) and submersing it in the water. Cook until their shells turn bright orange, about 2 minutes after the last crab goes in the pot. Transfer the crabs to a colander set in the sink, and run cold water over them. When they are cool enough to handle, remove the face of each crab (the strip on the front that encompasses the eyes and the mouth) using kitchen scissors. Then slip your thumb in the gap created between the top and bottom shells and pull off the top shell, exposing the feathery gills. Discard the top shell and the gills. (If you find any orange crab roe in there, reserve it and add it to the sauté pan with the garlic and chile flakes in the next step.) Turn the crab over and slide the tip of a knife beneath the spot where the cape of the shell tapers to a point; lift the bottom shell off and discard it. Crack each crab in two down the middle. Cleaning the crabs will take about 10 minutes. 

In a large sauté pan, melt the butter over medium heat until frothy. Add the garlic and chile flakes. Cook, stirring them around the pan, until the garlic is translucent and very fragrant, about 4 minutes. Add the hot sauce and cornstarch, whisk the contents of the pan to combine completely, and cook until the sauce just begins to bubble, about 2 minutes. Add the crabs to the pan and continue, turning them with tongs or a wooden spoon until all the crabs are evenly coated with the sauce. Cover, and cook just until the crabs are warmed through, about 3 minutes. Remove from the heat. 

Line a platter with the watercress or other green. Transfer the crabs to the platter and pile them in a mound. If the remaining sauce in the pan has broken at all, whisk it until it's emulsified again, and then pour the sauce over the crabs. Scatter green onions on top, if desired.