In a mixing bowl, whisk the soy sauce, onion, scallion, sesame oil, Sambal Oelek, garlic, scallion, and sesame seeds. Add the cubed tuna to the bowl and toss gently to coat. Let rest for 5 minutes at room temperature, then fold in the mango and avocado. (If necessary, you can also cover and refrigerate the poké for up to 2 hours, but wait to add the mango and avocado until just before serving.)