Place the olive oil into a small saucepan and place it over medium-low heat. While it is warming -- you do not want it to get too hot -- place the salt, garlic, cumin, and Scotch bonnet into a mortar and beat them up a bit with the pestle.
Add the warm oil to the mortar and let the seasonings steep for 10 minutes or longer. Whisk in the juices and vinegar. Season with pepper to taste.
I think the mojo is at its best after 24 hours. Store any unused mojo in the fridge.
Season the skirt steak with salt at the same time you start to make the mojo. If you plan to let the mojo meld overnight, let the skirt steak do the same with the salt by placing it uncovered on a cooling rack and into the fridge.
Shake or stir the mojo really well. Ladle the mojo over the skirt steak, making sure to get lots of peppers, garlic, and sour juice into the mix. Let the steak marinate no more than an hour. If you marinate them for too long, you wind up hiding the beef flavor behind the marinade flavors; the whole idea of this steak is to let the marinade be just strong enough to be a mystery.
While the steak is marinating, heat a skillet over high heat. Add a splash of oil to the hot pan and then the red onions. Quickly sear the onions, trying not to let them soften a whole lot. Remove the onions to a bowl and let them cool before stirring in the parsley.
Heat a large heavy-bottomed skilled over medium-high to high heat. Once the pan is very hot, pour a few glugs of oil into the pan. It should be enough to generously coat the bottom.
Drain the steaks of the marinade and pick off any stuck-on garlic and peppers.
Add the butter to the pan and swirl it into the oil. Gently lay the steaks into the sauté pan, being careful not to splash hot oil. Sear the steaks. Cook each side of the skirt steak until deeply caramelized.
Place the skirt steak onto a platter and shower them with the red onion and parsley mixture. Serve immediately with lime on the side for each diner to squeeze over their steak.