Finely chop basil and shallot in mini food processor.
Add butter, 2 tablespoons at a time, and process until blended, stopping occasionally to scrape down sides.
Transfer to small bowl; stir in lemon peel and season basil-shallot butter with salt.
Prepare barbecue (medium heat). Rub fish fillets on both sides with olive oil. Grill until fillets are just opaque in center, about 4 minutes per side.
Transfer fish to plates. Immediately spread some basil-shallot butter over fish.