Preheat oven to 350°. Combine pecans and panko; spread on a jelly-roll pan. Bake at 350° for 10 minutes. Place crumbs in a shallow dish. Place flour in a shallow dish. Combine water and egg in a shallow dish. Slice pork tenderloin crosswise into 8 slices. Place each pork slice between 2 sheets of heavy-duty plastic wrap; pound to 1/4-inch thickness using a meat mallet. Sprinkle pork with salt and pepper. Dredge in flour. Dip pork in egg mixture; dredge in crumbs. Melt 1 1/2 teaspoons butter in a large skillet over medium heat. Add half of pork to pan; cook 2 minutes on each side. Place pork on a jelly-roll pan. Repeat procedure with remaining pork and butter. Bake at 350° for 5 minutes. Serve with lemon wedges.