Heat the oven to 325ºF. Meanwhile, melt 2 tablespoons of the unsalted butter in a large oven-safe or cast iron skillet over medium-high heat. Add halved baby potatoes, onion powder, ¼ teaspoon kosher salt, and black pepper. Toss to combine, then arrange the potatoes cut-side down in a single layer. Cook undisturbed until beginning to brown on the bottom, 1 to 2 minutes.

Continue to cook, stirring occasionally, until well-browned and starting to soften, about 4 minutes. Turn off the heat. Transfer the potatoes to a rimmed baking sheet and arrange into a single layer. Place the baking sheet into the oven.

Wipe out the skillet. Melt 1 tablespoon of the unsalted butter in the skillet over medium-high heat. Add sirloin steak cubes in a single layer and season with the remaining kosher salt. Cook, flipping occasionally, until most of the sides are browned and the steak is just cooked through, 4 to 5 minutes.

Reduce the heat to low. Stir in the remaining 1 tablespoon unsalted butter, minced garlic cloves, and coarsely chopped fresh thyme leaves. Turn off the heat.

Transfer the potatoes to the skillet and toss to combine. Garnish with coarsely chopped fresh parsley leaves if desired.