Mix together cream cheese, mayonnaise, sour cream, onion powder, black pepper, lemon zest, lemon juice, and fresh dill.
Use a mandoline to slice the cucumber into extra-thin slices.
Spread a generous layer of the cream cheese mixture onto one side of each piece of bread.
Place a layer of cucumber slices onto a piece of bread over the cream cheese layer. Overlap the cucumbers slightly.
Assemble the sandwich by placing the second slice of prepared bread onto the cucumber layer (the cucumbers will be sandwiched between two cream cheese layers and the bread slices).
Cut the crusts from the sandwich to form a square. Cut the sandwich on the diagonal to form two triangular sandwiches.
Continue assembling sandwiches until you have used the whole loaf of bread.
Notes
Use English cucumbers rather than regular cucumbers - they are very large - you only need one to make sandwiches with a whole loaf of bread. They have thinner skin and don’t have to be peeled, and there are not many seeds.
The lemon and sour cream add a little zing to the cream cheese mixture. They also add flavor and break up the thick creaminess of the cream cheese.
You can use a hand mixer to quickly beat together the cream cheese mixture. Be sure the cream cheese is softened well before trying to mix.
A mandoline slicer is the best thing to slice very thin, even cucumber slices. You can get a huge number of slices from one English cucumber - close to 100 slices! Be careful when using a mandoline, as the blade is extremely sharp.
In the traditional English afternoon tea version, they butter the slices of bread before adding the filling as the butter acts as a barrier to the filling and prevents soggy bread.
The dill can also be used as a bit of fresh garnish on top of the sandwiches, particularly for a little extra fancy display.