Remove the steaks from the fridge at least 15 minutes before cooking.
Preheat the oven to 375 degrees F.
In a small bowl, mix together the sugar, garlic powder, pimenton, cayenne and 1 teaspoon salt. Drizzle the steaks with olive oil to lightly coat. Coat the steaks with the spice rub on all sides.
Heat a large cast-iron pan over high heat. Add the butter and let melt. Add the steaks and cook, turning to cook on all sides, until richly browned, about 10 minutes. Meanwhile, add the thyme bundle and garlic cut-side down. If the pan seems dry, add more butter. Tilt the pan to spoon the butter over the steaks as they cook. Once the steaks are browned and basted on all sides, place them on a sheet pan fitted with a wire rack. Transfer to the oven and cook through to desired temperature. I like my steaks medium rare! Allow the steaks to rest and the juices to settle at least 10 minutes before slicing.
Meanwhile, lower the heat to medium-low under the pan and add the onion rings. Allow to char on both sides, flipping once after about 1 minute. Set aside.
To a medium bowl, add the arugula, lemon juice and olive oil to lightly coat. Season with salt and pepper. Add the charred onion and toss.
Remove the meat from the bone and slice into 1- to 3/4-inch slices. Lay the steak on a serving platter with the bone on top and garnish with the arugula onion salad. Serve!