Rub backstrap with olive oil. Coat with blackening spices, wrap with plastic wrap and refrigerate for 2 to 3 hours. Remove from refrigerator, and let rest at room temperature for 20 minutes. While meat is resting, heat a heavy duty (preferably cast iron) skillet over high heat until it is very hot. "Smoking hot" is best, but make sure that you open a window or two for ventilation. Add meat to pan, and blacken on all sides, keeping the center of the meat as close to medium-rare as possible. Remove meat, and let it rest.
Prepare dressing. In a bowl, whisk together all dressing ingredients except olive oil. While whisking, add olive oil in a thin stream until emulsified.
In a large bowl, toss lettuce and cranberries with a light coating of dressing. Mound lettuce mix on plates. Arrange onion, cucumber and tomato around lettuce.
Thinly slice backstrap across the grain, and arrange on top of lettuce. Drizzle additional dressing over meat, and top with feta cheese.