Rinse lentils and drain. In a saucepan, put lentils and add 2 cups hot water. Simmer gently with lid tilted until tender. When the lentils are cooked, drain well.
Halve avocado, remove seed, peel, scoop avocado pulp and mush in a bowl. Add mayonnaise, plain yogurt, cumin powder, pinch of salt and granulated sugar to the bowl. Mix well. Then add minced onion, minced cucumber and cooked lentils. Mix well.
Put 1/6 of green lettuce leaves on one slice of bread, spread 1/6 of lentils mixture and put the other slice of bread to cover. Make 5 more sandwiches.