1. Heat oven to 375ºF. Place pork on rack in shallow roasting pan. Roast 30 to 35 minutes per pound until internal temperature reaches 150ºF.
2. Let stand 10 minutes before slicing.
For the chimichurri:
Place vinegar, cilantro, garlic, chiles, salt and pepper in a blender; purée at medium speed. After contents are pureed to a fine consistency, slowly add oil while blender is running. Adjust seasoning with salt and pepper.
For the pineapple relish:
Combine pineapple and onion; season with salt and pepper.
To serve, drizzle chimichurri onto sliced pork loin and top with pineapple relish. If desired, garnish with chopped cilantro.