Cut steak into 2–3 inch cubes. Rub with avocado oil and coat evenly in blackened seasoning. Let sit at room temperature for 10–15 minutes.
Peel and quarter potatoes. Soak in cold water for 10 minutes, then drain and set aside.
Boil potatoes in salted water until fork-tender while cooking the steak.
Heat a skillet over medium-high with 2–3 tablespoons avocado oil. Sear steak bites in batches, 2 minutes undisturbed per side, flipping once. Reduce heat and cook 1 more minute. Remove to a bowl.
Add 3 tablespoons butter and 1 tablespoon minced garlic to the skillet. Sauté until fragrant, then return steak to coat in garlic butter. Remove and tent with foil.
In the same skillet, add 2 tablespoons butter and remaining garlic. Sauté until fragrant, then slowly whisk in heavy cream. Simmer 3–5 minutes to thicken.
Stir in red pepper flakes, scallions, and parmesan cheese. Whisk until smooth. Adjust salt and pepper to taste.
Drain cooked potatoes. Mash with melted butter and warmed half and half. Add cheese, garlic powder, salt, and pepper. Mix until smooth.
To serve, spoon mashed potatoes onto plates. Top with steak bites and drizzle with parmesan cream sauce. Garnish with parsley if desired.