In a small bowl, whisk together mayonnaise, mustard, and lemon juice. Spread a layer of mayonnaise mixture onto each frozen bread slice. Top 6 bread slices with 2 pieces smoked salmon each. Cover with remaining 6 bread slices, spread side down, to make 6 sandwiches.
Using a serrated bread knife in a gentle sawing motion, trim and discard crusts from each sandwich. Cut each sandwich in half into 2 (approximately 2×1½-inch) tea sandwiches. Serve immediately, or cover with damp paper towels, place in an airtight container, and refrigerate for a few hours until serving time.
Recipe by TeaTime Magazine at https://teatimemagazine.com/dijon-smoked-salmon-tea-sandwiches-recipe/