Cut onions and tops in 1-inch lengths.
Cut green pepper in chunks and discard seeds.
Using metal blade, process onions and green pepper with on-off bursts until coarsely chopped.
Add cottage cheese, mustard, garlic salt, and horseradish and process with 3 on-off bursts.
Peel avocado, remove pit, and cut in chunks.
Distribute avocado over cheese and process with on-off bursts until avocado is mashed.
Drain tuna, break up with a fork, and distribute over cheese mixture.
Process with 3 on-off bursts to blend.