Cut fish into 4 pieces and season with salt and pepper.
On a work surface, arrange 3 slices bacon so they overlap slightly. Place a piece of fish at one short end and tightly wrap with bacon, cutting off any excess bacon. Repeat with remaining fish and bacon.
Whisk together mustard, honey, vinegar, and grapefruit juice; season with salt and pepper. Gradually whisk in oil.
Heat a large non-stick pan over medium-high.
Place fish, seam side down, in pan and cook until bacon is brown on all sides, about 9 minutes, turning occasionally.
Toss lettuce with dressing and grapefruit and serve with fish.