1. In the bowl of a mini food processor* add butter, 1 tablespoon of the mustard, parsley, rosemary (if using), garlic, ½ teaspoon of the salt, and pepper. Cover and pulse on high until ingredients are well minced.
2. Transfer 2 tablespoons of the butter mixture to a small bowl; set aside.
3. Using a paring knife cut shallow, ¼-inch deep, ¼-inch long slits, about 1-inch apart, all over the tenderloin. Fill each of the slits with the reserved 2 tablespoons of the butter mixture.
4. Preheat oven to 400°F.
5. In a 12-inch, oven-safe skillet heat olive oil over medium-high heat until shimmering. Add pork tenderloin to skillet. Cook 2 to 3 minutes each side or until lightly browned.
6. Place skillet in oven. Bake 25 minutes or until an instant-read thermometer reaches 140°F when inserted into the thickest part of the tenderloin.
7. Remove skillet from oven. Transfer tenderloin to a cutting board; let rest 10 minutes. 
8. Place skillet on burner. Add the remaining tablespoon of mustard and chicken stock, scraping up any browned bits on bottom of pan. Bring to boiling, reduce to a simmer. Simmer until sauce thickens, about 30 seconds.
9. Slice pork tenderloin into ½-inch medallions; serve with pan sauce and remaining 2 tablespoons of the butter mixture.

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