Preheat oven to 400°. Line a rimmed baking sheet with foil.
Arrange bacon slices in a single layer on prepared baking sheet.
Bake until bacon browns and begins to curl, 15 to 20 minutes, flipping bacon halfway through baking time. Drain bacon on paper towels.
In a small bowl, stir together mayonnaise, mustard, and parsley.
Using a 2-inch round cutter, cut 30 rounds from frozen bread slices, discarding scraps. To let thaw and to prevent bread from drying out during assembly, cover with damp paper towels, or store in a resealable plastic bag.
Spread a layer of mayonnaise mixture onto each bread round. On mayonnaise side of 15 bread rounds, layer a lettuce piece, a tomato slice, and a bacon slice. Top each with a remaining bread round, mayonnaise side down. Serve immediately, or cover with damp paper towels and serve within an hour.
Just before serving, garnish each tea sandwich with a parsley leaf, adhering to bread rounds with a little mayonnaise mixture, if desired.
Recipe by TeaTime Magazine at https://teatimemagazine.com/blt-holiday-tea-sandwiches/