Heat oven to 450°F. Pile potatoes and onions in the center of a rimmed baking sheet and place bacon on top. Roast for 10 minutes.

Meanwhile, in a small bowl, combine mayonnaise, mustard, and lemon zest. In a second small bowl, combine panko with oil, then fold in thyme. Season fish with 1/2 teaspoon pepper, then spread with mayonnaise mixture and sprinkle with panko.

Remove the baking sheet from the oven and reduce oven temperature to 425°F. Toss potatoes and onion mixture together, then spread in an even layer, arranging potatoes cut side down.

Nestle fish pieces among vegetables and roast until fish is opaque throughout and potatoes are golden brown and tender, 12 to 15 minutes.