Preheat oven to 400°.
Trim fat from pork tenderloin. Heat 1 teaspoon oil in a large, ovenproof Dutch oven over medium-high heat. Add pork; cook 8 minutes or until browned, turning occasionally. Remove pork from pan.
Add 2 teaspoons oil to pan. Add onion; sauté 5 minutes. Add pork and potatoes to pan; insert a meat thermometer into thickest portion of pork. Bake, uncovered, at 400° for 15 minutes. Add apples; bake an additional 20 minutes or until potatoes are tender and thermometer registers 160° (slightly pink). Remove pork and potato mixture from pan; keep warm.
Add cider, broth, Calvados, salt, and pepper to pan, scraping pan to loosen browned bits. Bring to a boil over high heat, and cook until reduced to 1 cup (about 8 minutes). Remove from heat. Stir in sour cream, stirring with a whisk until well-blended. Reduce heat, and simmer, uncovered, 1 minute. Cut pork into 1/4-inch-thick slices. Arrange 3 ounces pork on each of 6 plates; top each with 1 cup potato mixture and 1/4 cup sour cream sauce.
Note: For an alcohol-free version, simply increase the apple cider to 2 cups and omit the Calvados.