For the salad: Cut avocado into cubes, chop cilantro and cucumber; add to a large mixing bowl. Juice and zest lime, add to cucumbers. Cube mango and add to cucumbers along with brown sugar, grapeseed oil, vinegar and fish sauce. Toss well to coat evenly. Cover and refrigerate until ready to serve.
Add eggs to a saucepan and cover with cold water. Bring to a boil over high heat. Turn off heat and let sit for 10 minutes, covered. Cool and peel eggs then cut into large cubes; add to a large bowl.
Chop 1 stalk of celery and cut the remaining into sticks. Add chopped celery to eggs along with mayo and yogurt. Mix together, smashing some of the eggs. Spread an even amount onto 4 slices of bread. Top with remaining 4 slices of bread and cut into quarters.
Slice bell pepper. Serve sandwiches with cucumber salad, pepper slices and celery sticks.