Using a 3-inch square cutter, cut 6 squares from frozen bread slices, discarding scraps.
Using the same square cutter, cut 3 squares from turkey slices and 3 squares from cheese slices, discarding scraps.
Spread 1 tablespoon mustard onto each of 3 bread squares. Top each with a cheese square and a turkey square. Spread 1 tablespoon mayonnaise onto each of remaining 3 bread squares. Place on top of turkey layer, mayonnaise side down.
Preheat a nonstick skillet over medium heat. Melt butter in pan. Add sandwiches to pan, and toast on both sides until golden brown and cheese begins to melt, approximately 1 minute per side. Remove from pan, and let cool slightly.
Using a large sharp knife in a downward motion, cut sandwiches in half diagonally to create 2 triangles. Serve immediately.
Recipe by TeaTime Magazine at https://teatimemagazine.com/turkey-and-american-grilled-cheese/