Step 1: Preheat your oven to 200°C (400°F). Pat the beef tenderloin dry with paper towels to ensure a good sear.
Step 2: Season the beef generously on all sides with salt and black pepper.
Step 3: Heat olive oil in a large, oven-safe skillet over medium-high heat until shimmering. Carefully place the tenderloin in the hot skillet and sear for 2-3 minutes per side, creating a golden-brown crust.
Step 4: Transfer the skillet with the seared tenderloin to the preheated oven. Roast for 20-30 minutes, or until a meat thermometer inserted into the thickest part reads 130-135°F (54-57°C) for medium-rare.
Step 5: Remove the tenderloin from the oven and transfer it to a cutting board. Tent loosely with foil and let it rest for at least 10-15 minutes before slicing.
Step 6: While the beef is roasting, wash and cut the potatoes into even 1-inch (2.5 cm) cubes or wedges.
Step 7: In a large bowl, melt the butter. Add the chopped potatoes, fresh rosemary, and a pinch of salt and pepper. Toss until the potatoes are evenly coated.
Step 8: Spread the seasoned potatoes in a single layer on a baking sheet. Roast them for 25-35 minutes, flipping halfway through, until golden brown and tender.
Step 9: Slice the rested beef tenderloin against the grain and serve immediately with the crispy rosemary potatoes.