Mornay Sauce:
To prepare sauce, melt butter in a medium saucepan. Add flour, stirring well with a whisk.
Gradually add milk, pepper, and salt, stirring constantly with a whisk until blended.
Place over medium heat; cook until thick (about 5 minutes), stirring constantly.
Remove from heat; stir in cheese.
Sandwich:
Vacuum seal the turkey breasts with salt and pepper
Preheat the sous-vide machine to 140
Cook the turkey breasts in the sous-vide machine for 2.5 hours
Preheat the BGE to 350
Cook the bacon, tomatoes, and bread
Slice the turkey
Place bread in a cast iron pan
Add the sliced turkey and sliced tomatoes to the bread
Cover with the Mornay Sauce
Place the cast iron pan in the BGE for 10-15 minutes until the sauce is bubbly and slightly browned
Add the bacon, chopped chives, and serve