Slice bagels in half horizontally and toast in a toaster; slice onion and tomato.
On the stove, warm a skillet to medium heat.
Make 4 small “sandwich” piles by layering turkey, cheese, onions, and tomatoes (in that order) on a cutting board. Transfer piles to the hot skillet, keeping turkey on the bottom.
Cover with a lid and cook for 4 minutes until warmed through and cheese is melted.
Meanwhile, spread mustard onto bagels.
Add warmed sandwich piles to the bottoms of the bagels; top with a lettuce leaf and then the tops of the bagels, forming 4 sandwiches. Cut each in half if desired. Serve with orange wedges.