Slice the red onion into thin half-rings and the tomato into ¼-inch slices.
Mash the avocado in a bowl. Add the lemon juice and ¼ teaspoon of salt. Stir to combine.
Toast the bread. Add the mashed avocado to one side of the bread and top with the red onion. Add the hummus to the other slice of bread and top with the sliced cucumber, radish, tomato with remaining salt and pepper, and sprouts.