Combine the tuna, shallot, ginger, jalapeno (if using), sesame oil, soy sauce, and cilantro in a medium bowl and toss gently to combine. Cover with plastic wrap and refrigerate for 1 to 2 hours before serving.
Just before serving, fold in the avocado. Season with salt and pepper to taste. (I found that it was flavorful enough without salt and pepper, but season according to your taste.)