Cut the cucumber in half lengthwise and use a spoon to scrape the seeds out. Grate the cucumber (leave skin on), toss with 1/2 tsp salt and place in a sieve (set sieve over bowl). Place something heavy over top of the cucumber to help remove the excess moisture. Let sit for 20 minutes (about 3 Tbsp of water will be released!). Use as is, or rinse quickly with water to remove salt and pat dry.
Place cucumber in a dish with peppers, cilantro, oil, lime juice and 1 garlic. Stir and season further with salt and pepper if desired. Let sit for 20 minutes for flavors to blend.
Sauté shrimp in a bit of oil and garlic until fully cooked. Slice in half lengthwise, keep warm or allow to cool to room temperature.
Brush flatbread with a bit of oil and broil until toasted (if you made the bread thicker, slice in it half horizontally).
Once the flat bread has cooled, spread over some of the cheese, then cut your bread up into small squares or rectangles (two-bite serving sizes), then assemble with all the toppings. This makes a mess-free clean-up, and you can artfully stack each crostini individually.