How to Make It
Combine the Serranos, onion, garlic, mustard, vinegars, salt and pepper in a blender and mix well. With the motor running, slowly add the oil and blend until emulsified. Pour into a bowl and stir in the chopped basil.
Heat grill to medium-high. Brush both sides of each fish with the oil and season with the salt and pepper, including the cavities. Stuff the cavity of each fish with some of the basil. Grill fish until slightly charred on one side, 4 to 5 minutes. Using a heavy-duty spatula, carefully turn the fish over, close the cover and continue grilling until the fish is just cooked through, 6 to 7 minutes. Remove from the grill and immediately drizzle fish with 2 tablespoons of the vinaigrette.