Pat steak cubes dry with paper towels, then press the crushed peppercorns and salt onto all sides of the meat.

Heat olive oil and 1 tablespoon butter in a large skillet over high heat until very hot and shimmering.

Add steak bites in a single layer (work in batches if needed) and sear for 2-3 minutes per side for medium-rare, then remove to a plate.

Reduce heat to medium, add remaining butter and shallots to the same skillet, and cook for 2 minutes until softened.

Remove skillet from heat, add cognac, then return to heat and let it simmer for 1 minute to reduce.

Stir in heavy cream and Dijon mustard, scraping up any browned bits from the bottom of the pan, and simmer for 3-4 minutes until sauce thickens.

Return steak bites to the skillet, toss in the sauce to coat, garnish with parsley, and serve immediately.