Prep the steak: Pat the steak cubes dry with paper towels. Season generously with salt and pepper on all sides. Let sit at room temperature for 10–15 minutes while you prep other ingredients.
Heat the pan: Set a large heavy skillet (cast iron works best) over medium-high heat.
Add olive oil and let it shimmer.
Sear in batches: Add half the steak in a single layer without crowding. Cook 1–2 minutes per side until browned with a nice crust and medium-rare in the center. Transfer to a plate and repeat with the remaining steak.
Do not overcook.
Add butter and garlic: Lower heat to medium. Add butter to the empty pan. Once melted, stir in the minced garlic and cook 30–45 seconds until fragrant.
Scrape up browned bits for extra flavor.
Build the sauce: Pour in the heavy cream. Whisk in Italian seasoning, Dijon (if using), and red pepper flakes. Simmer gently for 2–3 minutes to thicken slightly.
Add Parmesan: Reduce heat to low.
Add Parmesan a handful at a time, whisking until smooth and creamy. If the sauce feels too thick, loosen with a splash of water or stock; if too thin, let it simmer a bit longer.
Brighten: Stir in lemon juice and taste. Adjust salt and pepper as needed.
The sauce should be rich, garlicky, and balanced.
Toss the steak: Return the steak (and any juices) to the pan. Toss to coat and warm through for 30–60 seconds. Do not simmer the steak in the sauce for long, or it may overcook.
Finish and serve: Sprinkle with chopped parsley.
Serve immediately over mashed potatoes, rice, pasta, or roasted vegetables.