Preheat the oven to 400 degrees . In a medium saucepan, melt 2 tbsp. butter over medium heat. Whisk in the flour and cook for 2 minutes. Whisk in the milk and bring to a boil, whisking constantly. Remove from the heat and stir in 1 cup cheddar; season.
In a large skillet, melt 1 tbsp. butter over medium heat and add the pork; cook until done, about 5 minutes. Transfer to a plate. Add bok choy stems and onion to the skillet; cook until tender. Stir in the pork, bok choy leaves and zest; season.
Grease an 8-inch square baking dish and spread with 1/2 cup sauce. Layer the dish with 9 wrappers (dipped in warm water first), one-third of the meat, 1/2 cup sauce and another wonton layer. Repeat twice. Top with remaining sauce.
Cover the dish with foil and bake for 20 minutes. Uncover, sprinkle remaining cheddar on top and bake until golden, about 20 minutes. Let cool before cutting.