Prepare macaroni and cheese according to package directions.
In a medium ovenproof pan or dish (about 4 cups/1 L), spread cooked macaroni and cheese in 1-in. (2.5-cm) layer. Top with Swiss cheese, then sprinkle panko, green onion and bacon over top. Broil on top rack of oven until golden and bubbly, and bacon edges are crispy, 2 to 4 min.