Trim off root ends of lemongrass; remove all tough outer leaves. Cut off tender lower 4 to 6 in. (10 to 15 cm) of stalks, and chop. Discard tops. Purée chopped lemongrass and water in blender until smooth, about 2 minutes

Add lime juice, garlic, honey, soy sauce, pepper flakes, salt and canola oil, and blend until smooth. Reserve 1/3 of marinade. Place pork in large resealable plastic bag; add 2/3 of marinade. Marinate in refrigerator for at least 4 hours or overnight.

Preheat grill to medium and brush with canola oil. Remove tenderloin from marinade and shake off excess. Discard marinade.

Grill until cooked through, about 6 to 8 minutes per side. Baste pork with reserved marinade halfway through cooking time. Cook until 140 °F to 145 °F (60 °C to 63 °C) for medium-rare to medium or 150 °F to 155 °F (66 °C to 68 °C) for medium to medium-well. Remove from grill, and allow to rest for 5 minutes before slicing. Sprinkle with cilantro and serve.