1. Place chiles, onion, garlic, mustard, vinegar, salt and pepper in a blender and blend until combined. With the motor running, slowly add the oil and blend until emulsified. Pour into a bowl and stir in the chopped basil.
2. Heat grill to medium-high heat. Brush both sides of the fish with the oil and season each with 1 tsp. of salt and pepper, including the cavity. Stuff each fish with some of the basil. Grill until slightly charred on one side, about 4 to 5 minutes. Carefully turn the fish over and grill until cooked through about 6-7 minutes. Remove from grill and drizzle each piece of fish with 2 Tbsp. of vinaigrette.
Note: This recipe is easily adaptable depending on the ingredients you have available. Substitute another white fish, use fillets, or even chicken. If you don't like serrano chiles, substitute a different flavor that you prefer.