Spread mustard over 1 side of each bread slice. Layer 4 bread slices, mustard sides up, with turkey and cheese slices. Top with remaining bread slices, mustard sides down.
Whisk together eggs and milk in a shallow dish. Dip both sides of each sandwich into egg mixture.
Melt 1 Tbsp. butter in a large nonstick skillet over medium heat; cook 2 sandwiches in skillet 3 to 4 minutes on each side or until lightly browned. Repeat procedure with remaining butter and sandwiches. Serve immediately with blackberry preserves. Dust with powdered sugar, if desired.
For an easy side, pulse 1 (28-oz.) can Italian-seasoned tomatoes in a food processor until finely diced. Stir together tomatoes, 1 (26-oz.) can tomato soup, 1 (32-oz.) container chicken broth, and 1/2 tsp. freshly ground pepper in a Dutch oven. Cook over medium heat, stirring occasionally, 10 minutes or until thoroughly heated.