Start the mashed potatoes first — they take about 25 minutes and you want them ready when the steak is done.
In a bowl, toss 1½ pounds of steak cubes with a drizzle of vegetable or olive oil, 1½ teaspoons smoked paprika, ¾ teaspoon garlic powder, ¼ teaspoon onion powder, 2 teaspoons Italian seasoning, 1 teaspoon kosher salt, and ½ teaspoon black pepper. Let the seasoned steak sit for 15 minutes at room temperature.
Heat a 10-inch cast iron skillet over high heat until very hot. Add a drizzle of vegetable or olive oil. Sear the steak bites in batches — do not crowd the pan — for about 2 minutes per side, or longer depending on your preferred doneness, until a golden crust forms. Remove the steak from the pan and set aside.
Lower the heat to medium-low and let the pan cool slightly. Add 4 tablespoons unsalted butter, 6 minced garlic cloves, and the finely minced half shallot. Sauté for a few seconds, just until fragrant.
Return the seared steak bites to the pan and toss to coat in the garlic butter. Transfer the steak bites and half of the garlic butter to a bowl. Leave the remaining garlic butter in the pan.
Pour 1 cup heavy cream into the skillet with the remaining garlic butter. Bring to a low simmer for a few seconds, then turn off the heat. Stir in ⅓ cup freshly grated parmesan and ½ tablespoon chopped Italian parsley until the sauce thickens. Season with salt and pepper to taste.
Spoon the mashed potatoes into bowls or onto plates. Top with the garlic butter steak bites, drizzle the creamy parmesan sauce over everything, and garnish with extra chopped parsley.