Start by cutting the steak into 2-3 inch pieces (they’ll shrink slightly during cooking). Rub them all over with avocado oil, then season generously with the homemade blackened seasoning and let them sit for 10-15 minutes at room temperature so the flavours set in.
Peel and quarter the russet potatoes, then soak them in a bowl of cold water for 10 minutes (this removes excess starch). Drain and then boil in salted water until they are fork tender while you cook the steak.
Heat 2-3 tablespoons avocado oil in a large skillet over medium-high heat. Add steak in a single layer without overcrowding, sear without moving for ~2 minutes until golden, flip once, cook ~1 minute, then reduce heat to low and cook another ~1 minute for medium. Remove and tent with foil to keep moist.
In the same pan, add 2 tablespoons butter and 1 tablespoon minced garlic, sauté until fragrant, then return the steak and toss to coat on all sides in the garlic butter. Remove and set aside.
In that same pan, add another 2 tablespoons butter and the remaining minced garlic, sauté, then whisk in heavy cream and let it simmer for 3-5 minutes until it reduces. Meanwhile, drain the potatoes, add melted butter, warmed half-and-half, shredded cheese and garlic powder, salt and pepper, and mash until smooth. Add red pepper flakes, scallions and parmesan to the sauce, whisk until thickened, taste and adjust seasoning. Serve steak over the mashed potatoes and spoon the cream sauce on top.