Herb-Rubbed Pork Tenderloin with Fingerling Potatoes

0 Preheat oven to 500°F / 260°C.
1 In a baking dish or bowl that is large enough to hold the pork, mix together all ingredients for spice rub.
2 Rub pork tenderloin with 1 Tablespoon oil (this will help the spices to stick). Roll pork in spices until evenly coated. Refrigerate until ready to use. 
3 When oven is heated, toss potatoes with 1 Tablespoon oil, salt, and dried oregano. Spread out on a large sheet pan and cover tightly with foil.
4 Roast potatoes, covered, for 15 minutes. 
5 While potatoes roast, heat 1 Tablespoon oil in a large skillet over medium heat.
6 When oil is shimmering, add pork and sear on all sides until golden brown. 
7 Move skillet off the heat but keep it on the stove (you’ll use it for the pan sauce).
8 Remove potatoes from the oven and remove foil. (Reserve the foil for tenting the pork later.)
9 Turn oven’s heat down to 425°F / 218°C
10 Flip potatoes with a spatula and move them out to the sides of the baking sheet. If the potatoes stick to the bottom of the pan, drizzle with a bit more oil as you toss them. 
11 Nestle pork in the center of the potatoes. 
12 Return sheet pan to the oven and continue baking together until the potatoes are tender and the pork reaches an internal temperature of 145°F. They should finish cooking at around the same time, 12 to 20 minutes more.
13 While pork and potatoes roast, make the pan sauce. Add butter to the skillet you used to sear the pork. Move it back over medium heat. When butter melts, add in shallots and cook until tender, 2 to 3 minutes. Sprinkle flour over top and stir until no dry spots remain. Slowly add in stock or water while scraping up any browned bits on the bottom of the pan. Bring to a simmer. Simmer sauce until thickened, about 5 minutes. 
14 Move sauce off heat and whisk in lemon juice, mustard, and parsley.
15 When pork is done cooking cover it with foil and let rest for 8 to 10 minutes. (This is the key to juicy, tender pork - don’t skip this step.)
16 Slice pork and serve with potatoes. Serve sauce over top.