Season steaks generously on all sides with salt and pepper; allow to reach room temperature for 30 minutes.
Heat oil in a heavy cast-iron skillet over high heat until it reaches its smoking point.
Place steaks in the skillet and sear for 3-4 minutes per side until a deep golden-brown crust forms.
Reduce heat to medium-high; add butter, smashed garlic, and rosemary sprigs to the skillet.
Tilt the pan and baste the steaks continuously with the foaming butter for 2-3 minutes until internal temperature reaches 130°F.
Remove steaks to a cutting board and rest for 10 minutes.
Discard rosemary and garlic from the skillet, leaving approximately 1 tablespoon of fat in the pan.
Add heavy cream and garlic powder to the skillet over medium heat, scraping the bottom to deglaze the browned bits.
Whisk in Parmesan cheese until melted and the sauce has thickened, approximately 2 minutes.
Slice steaks against the grain and serve topped with the Parmesan cream sauce and fresh parsley.