Preheat oven to 450°.
Combine the first 4 ingredients; rub over pork. Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add pork; cook 5 minutes, browning on all sides. Wrap handle of skillet with foil; place pan in oven. Bake at 450° for 20 minutes or until a thermometer registers 155° (slightly pink). Remove pork from pan, and let stand for 10 minutes. Cut the pork crosswise into 1/2-inch-thick slices.
Heat oil in a Dutch oven over medium heat. Add onion; cook 5 minutes, stirring frequently. Add prosciutto and garlic; cook 1 minute, stirring constantly. Add spinach, tossing to combine. Add water, vinegar, and honey; cook 2 minutes or until spinach wilts, tossing frequently. Stir in piquillo peppers. Arrange about 1/3 cup spinach mixture on each of 6 plates; top each serving with 3 ounces pork.