Bring a large pot of heavily salted water to a boil. Cook the fettuccine 1 minute shy of package directions. Reserve 1 cup of pasta water before draining.

Pat the steak cubes dry, season with salt and pepper. Heat olive oil in a heavy skillet over high heat until shimmering. Sear steak in a single layer 60 to 90 seconds per side until golden but still pink inside. Remove to a plate.

Reduce heat to medium. Add 3 tbsp butter and minced garlic. Cook 30 seconds until fragrant but never browned.

Pour in heavy cream, soy sauce, and red pepper flakes. Simmer 1 minute. Add parmesan and remaining 3 tbsp butter. Stir until silky.

Add drained pasta directly to the skillet. Toss 1 to 2 minutes, splashing in pasta water as needed so sauce clings to every strand.

Return steak and resting juices to the skillet. Toss gently to coat. Taste and adjust salt.

Plate and top with fresh parsley, extra parmesan, and a pinch of red pepper. Serve immediately.