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.