Step 1: Prepare the Cucumber Ribbons Wash the cucumber well. Using a vegetable peeler or a mandoline slicer set to a thin setting, peel the cucumber lengthwise into long, wide ribbons. Aim for ribbons about 1/8 inch thick—thin enough to bend easily but thick enough to hold their shape. You should get about 16 to 20 ribbons per cucumber. Stop peeling when you reach the seedy core, which is too watery to use. Pat the ribbons dry with a paper towel to remove excess moisture.
Step 2: Make the Avocado Filling Cut the avocado in half, remove the pit, and scoop the flesh into a small bowl. Add the lime juice and salt. Mash with a fork until the mixture is creamy but still has small chunks for texture. Fold in the diced red bell pepper and the chopped herbs, if using.
Step 3: Assemble the Rolls Lay one cucumber ribbon flat on a clean cutting board. Spoon about 1 to 2 teaspoons of the avocado mixture in a thin line along one short edge of the ribbon. Gently roll the cucumber ribbon over the filling, tucking it slightly, and continue rolling into a tight cylinder. Repeat with the remaining ribbons and filling. The rolls should hold together on their own, but you can secure them with a toothpick if needed.
Step 4: Garnish and Serve Place the finished rolls seam-side down on a serving platter. Sprinkle the tops with sesame seeds and a tiny pinch of flaky sea salt if desired. For extra freshness, add a small cilantro leaf on top of each roll.