Heat grill over med-high. Pat salmon dry and sprinkle all over with 1/4 tsp salt.Advertisement
Whisk mustard with oil and 1/4 tsp salt. Brush top of fish with half the mustard mixture and grill, top down, until well marked, 3 to 5 min.
Brush second side of salmon with remaining mustard mixture. Flip and cook until temp reaches 120°, 3 to 5 min more. Top salmon with scallions and serve with lemon halves.