Heat broiler with rack 6 inches from element. Toast bread on both sides until firm and pale gold.
Melt butter in a saucepan over medium heat. Whisk in flour; cook 1 minute without browning.
Gradually whisk in warm milk; simmer 4 to 5 minutes until sauce coats a spoon.
Remove from heat; whisk in 3/4 cup Pecorino, salt, and nutmeg until smooth.
Place each toast in a broiler-safe shallow dish; cover with 3 ounces turkey and 2 tomato slices.
Spoon Mornay over sandwiches; scatter remaining Pecorino and cross 2 bacon slices over each.
Broil 3 to 5 minutes until sauce bubbles and browns in spots and turkey reaches 165°F.
Rest 2 minutes before carrying hot dishes to table.