Combine first 3 ingredients; rub evenly over pork. Place pork on a broiler pan coated with cooking spray. Bake at 425° for 20 minutes or until a thermometer registers 160° (slightly pink). Remove pork from oven; let sit 5 minutes before slicing.
While pork cooks, heat oil in a large nonstick skillet coated with cooking spray over medium heat. Add onion; cover and cook 10 minutes or until golden brown, stirring frequently. Uncover and cook 1 minute, stirring constantly.
Fill each taco shell with about 2 ounces pork, 3 tablespoons sautéed onion, 1 tablespoon tomato, and 1 teaspoon green onions.