Preheat oven to 400°F (205°C).
Pat the pork tenderloin dry with paper towels.
In a small bowl, mix olive oil, garlic, rosemary, Dijon mustard, and lemon zest. Rub evenly over the pork. Season with salt and black pepper.
Heat a heavy skillet over medium-high heat. Add olive oil.
Sear the pork tenderloin for 2–3 minutes per side, until nicely browned on all sides.
Transfer the skillet to the oven and roast for 12–18 minutes, or until the internal temperature reaches 140–142°F (60–61°C).
Remove from oven, tent loosely with foil, and rest for 5–10 minutes until the temperature rises to 145°F (63°C).
(Optional Sauce) In the same skillet, saute shallot for 30–60 seconds. Deglaze with stock or wine, simmer until slightly reduced, whisk in butter and Dijon.
Slice pork against the grain and serve with pan sauce if desired.