Mix the garlic powder, onion powder, cayenne pepper and 2 tablespoons salt together in a small bowl. Rub all sides of the steak with the seasoning blend. Allow to sit for 10 minutes until ready to cook.
For the asparagus: Bring a pot of salted water to a boil. Prepare an ice bath in a separate bowl and add a large pinch of salt. Add the asparagus to the boiling water and blanch, cooking until crisp-tender, 1 to 2 minutes. Remove the asparagus and immediately shock in the ice bath to stop the cooking. Drain and set aside.
For the steak: Coat a large cast-iron skillet with a generous layer of olive oil and let heat over medium high until shimmering and starting to smoke. Sear the steaks on all sides until a brown crust forms, about 8 minutes total.
Carefully pour out the fat from the skillet. Nestle the garlic halves cut-side down next to the steak and drizzle in about 1 tablespoon olive oil. Transfer the skillet to the oven and roast until the internal temperature of the steak reaches 120 degrees F for medium-rare, about 12 minutes. Remove the steaks and garlic from the skillet and allow to rest for 10 minutes.
For the potatoes: Meanwhile, add the potatoes to a small pot of salted cold water, making sure they are submerged. Bring to a boil over medium-high heat, then reduce to a simmer. Cook until the potatoes are fork-tender, 10 to 12 minutes. Drain and return to the pot.
For the compound butter: Combine the butter, parsley, mustard, lemon zest and juice and a big pinch of salt. Mix until evenly incorporated. Set aside.
For the potatoes: Heat the heavy cream in a small saucepan over medium-low until warm. Using a hand mixer, beat the potatoes until smooth. Add the cream a splash at a time, the 4 tablespoons of butter a pat at a time, and the horseradish and beat until smooth but not soupy (you may have extra cream leftover). Season with salt, then add the 2 tablespoons cold butter.
For the asparagus: Coat a medium saute pan with olive oil and heat over medium heat. Add the shallots and a pinch of salt and red pepper flakes and cook until translucent, about 2 minutes. Add the asparagus and cook until heated through, about 1 minute. Season with salt.
Slice the steak on the bias into thick pieces. Smear with the compound butter and any extra drippings from the skillet and return to the oven in the skillet to melt the butter, about 30 seconds. Arrange on a plate and serve with the roasted garlic, asparagus and potatoes.