Mash the avocados in a small bowl. Mix them with cilantro, red onions, dried red pepper flakes, lime juice, cumin, salt, and pepper. Cover with plastic wrap and place in the refrigerator to chill while you prepare the rest of the ingredients.(Alternatively, you can use store-bought guacamole, but I like to make my own.)
Slice the cucumber about 3/4 of an inch thick.
Toss the shrimp in olive oil and Cajun seasoning until evenly coated. Transfer the shrimp to a skillet and cook 2-3 minutes on each side until blackened.
Arrange the cucumber slices on the serving tray and top each one with guacamole. Place a thinly sliced piece of radish on top of the guacamole.
Place one shrimp on top of each guacamole topped cucumber slice. Squeeze a bit of fresh lime juice on top and finish with a drizzle of balsamic vinegar or balsamic reduction.
If you have a hard time getting them to stay together, stick toothpicks through them. Toothpicks are also convenient for serving bite-sized appetizers.