For the Rosemary Chickpea Salad: Place all ingredients in a mixing bowl and mash together with a fork until the chickpeas are broken down into smaller chunks (not mushy). Taste and season with more salt, pepper, or garlic powder if desired.
For Assembly: If you're eating this sandwich right after making it, I suggest toasting the buns for a nice touch. If you're packing it away for later, do not toast, as they may get stay after being packed.
I build this sandwich upside down to help keep it stable! Start with the two bottom One Bun halves, top them with avocado, then red onion, tomatoes, cucumber, and arugula. Divide chickpea salad between both stacks, then spread dijon mustard on the remaining buns. Top the stack and serve! If you're packing for later, cut the sandwiches in half with a serrated knife, then place in a container and refrigerate until ready to serve.