Combine parsley, garlic, hot pepper sauce, salt and pepper in food processor or blender until parsley is finely chopped. With motor running, gradually blend in olive oil and lemon juice until mixture resembles pesto. (Make-ahead: Chimichurri Sauce can be prepared up to 12 hours in advance and kept refrigerated for up to 2 days.)
Set aside 3 tbsp (45 mL) of sauce and reserve; cover and refrigerate. Pierce meat numerous times with fork. Place in sealable freezer bag with the remaining sauce; marinate in refrigerator for 12 to 24 hours. Discard marinade.
Grill Medallions using medium-high heat, 10 to 12 minutes, turning at least twice for medium-rare doneness 145°F (63°C). Serve each Medallion with a spoonful of reserved sauce or Chimichurri Salsa (recipe follows).
Chimichurri Salsa: Add 1 large plum tomato, diced and 2 tsp (10 mL) chopped fresh cilantro (if desired) to the reserved portion of Chimichurri Sauce.