Preheat the oven to 375 degrees F. Set a wire cooling rack inside a sheet pan.
Remove the steak from the fridge and bring to room temperature, about 45 minutes.
Mix together the garlic powder, onion powder, salt and cayenne in a small bowl. Sprinkle the entire steak with the rub on both sides, adding more salt if necessary.
Add 2 tablespoons olive oil to a large cast-iron skillet and heat over medium high. When the skillet is lightly smoking, sear the steak until well browned, 3 to 4 minutes per side. While searing, use a spoon to baste the steak with the oil from the pan. Turn off the heat and place the garlic halves cut-side down in the skillet and sprinkle the thyme sprigs around the steak. Transfer the skillet to the oven to finish cooking until the internal temperature reaches 135 degrees F when measured with an instant-read thermometer not touching bone, about 8 minutes. Remove the steak and garlic halves to the cooling rack to rest for 10 minutes.
Meanwhile, bring the potatoes and remaining 4 garlic cloves to a boil in a medium pot of heavily salted water. Reduce to a simmer and cook until the potatoes are fork tender, 10 to 12 minutes. Drain the potatoes and return them to the empty pot.
Heat a skillet over medium-high heat and coat with olive oil. Add the shallots and sweat until soft, about 3 minutes. Add the sliced mushrooms and a pinch of salt. Continue cooking, tossing occasionally to ensure that the mushrooms do not stick to the skillet, until the mushrooms are tender and browned, about 10 minutes. Add the red wine and simmer until reduced by half, about 3 minutes. Stir in the demi-glace and reduce until the mixture is thick, saucy and coats the back of a spoon, about 4 minutes.
Using an electric hand mixer, beat the potatoes until smooth. Add 2 pats of butter and a splash of the hot heavy cream. Continue mixing. Repeat, adding more butter and cream 2 more times and finishing with a pinch of salt.
Cut the bone off the steak and slice the meat on the bias, keeping the slices together (reserve the bone). Fan the steak slices on a serving plate and garnish with the bone and the roasted garlic halves. Spoon the mushroom sauce around the edge of the steak and garnish the steak with chives. Serve with a small ramekin of extra sauce. Serve the mashed potatoes in a separate bowl garnished with a small pat of butter and more chives.