Drain and rinse the chickpeas thoroughly, then pat dry with a paper towel to remove excess moisture.
Add chickpeas to a large bowl and mash with a fork or potato masher until roughly broken down, leaving about one-third of the chickpeas partially whole for texture.
Scoop avocado flesh into the bowl and mash it directly into the chickpea mixture until a creamy, cohesive base forms.
Add Dijon mustard, lemon juice, soy sauce, garlic powder, salt, and black pepper. Stir well to combine and taste for seasoning.
Crumble the nori sheet into fine flakes and stir into the filling along with the diced celery and minced red onion.
Let the mixture rest for 5 minutes so the flavors meld together.
Toast bread if desired, spread filling generously onto one slice, layer on toppings, close the sandwich, and cut diagonally before serving.
