Pat the sirloin steak cubes dry with paper towels. In a medium bowl, toss the steak cubes with olive oil, salt, and black pepper until evenly coated.

Heat a large skillet over medium-high heat until smoking. Add the seasoned steak bites in a single layer, ensuring not to overcrowd the pan. Cook for 2-3 minutes per side for medium-rare, or longer for desired doneness, until a deep brown crust forms. Work in batches if necessary.

Once all steak bites are cooked, reduce heat to medium. Add the melted butter and minced garlic to the skillet. Sauté for 1 minute, tossing the steak bites to coat them in the garlic butter. Stir in the chopped fresh parsley. Remove steak bites from the skillet and set aside.

In a small saucepan over medium heat, melt 2 tablespoons of unsalted butter. Add the all-purpose flour and whisk continuously for 1 minute to create a roux.

Gradually whisk in the warm whole milk until smooth. Bring the sauce to a gentle simmer, whisking constantly, until it thickens slightly (about 2-3 minutes). Stir in the heavy cream, grated Parmesan cheese, salt, black pepper, and optional nutmeg. Continue to simmer and whisk gently for another 1-2 minutes until the cheese is melted and the sauce is creamy and smooth.

Divide the garlic butter steak bites among plates. Drizzle generously with the creamy Parmesan sauce. Garnish with extra fresh parsley if desired. Serve immediately with your favorite sides.