Mix the butter, Dijon and fresh basil in a small bowl.
Preheat the BBQ or broiler; brush the grill or broiler pan with olive oil. Season the fish with salt and pepper. Grill fish just until cooked through, 3 to 4 minutes per side.
Transfer the swordfish to plates and top each with a spoonful of the mustard basil butter. Garnish with basil sprigs.
Notes
Use gluten-free Dijon mustard if needed.
Use less butter topping to reduce fat and calories.