Combine shallots, rice vinegar, sumac, and ¼ tsp. salt in a small bowl. Massage sumac into shallots, and let sit for a minimum of 20 minutes, or up to 3 hours.
Toast sesame seeds in a dry skillet over medium heat for 3 to 5 minutes, or until lightly browned and aromatic. Stir often and watch closely to make sure they do not burn. Set aside.
Arrange cucumber and avocado chunks on a large plate or serving platter. Season with remaining ¾ tsp. salt, and drizzle with olive oil. Scatter shallots and residual rice vinegar over cucumber and avocado chunks, and garnish with toasted sesame seeds and fresh dill.
