Preheat oven to 350ºF (175ºC).

Tie the roast with kitchen twine if needed and season generously with salt and pepper.

In a small bowl, combine chopped rosemary, garlic, and olive oil to form a paste.

Heat vegetable oil in a cast iron skillet over medium-high heat. Sear all sides of the roast (about 5–8 minutes total).

Remove skillet from heat. Brush the rosemary-garlic paste evenly over the roast.

Transfer the skillet to the oven and roast for 1 to 1.5 hours, or until internal temp reaches 135ºF for medium-rare.

Remove from oven and let rest for 10 minutes (temperature should rise to 145ºF).

While resting, sauté mushrooms in 2 tbsp butter over medium heat until browned and liquid evaporates (about 5 minutes). Season with salt and pepper.

Remove roast from skillet. Add stock to pan and deglaze, scraping browned bits. Simmer until slightly thickened.

Stir in mushrooms and remaining 2 tbsp butter until sauce is silky. Return roast to skillet and spoon sauce over the top.

Garnish with fresh rosemary if desired. Serve sliced with mushroom sauce.