In a large mixing bowl, combine the mashed avocado and rinsed chickpeas. Using a fork, gently mash them together, leaving some chickpeas whole for texture.
Fold in the finely chopped red onion and diced celery into the bowl, combining them evenly.
In a separate small bowl, whisk together the fresh lemon juice, either mayonnaise or Greek yogurt, Dijon mustard, garlic powder, salt, and pepper until well blended.
Pour the dressing over the avocado and chickpea mixture. Carefully fold everything together until well coated. Taste and adjust seasoning if needed.
If desired, toast the slices of whole grain bread until golden brown.
Lay leafy greens on the bottom two slices of bread. Spoon the avocado chickpea mixture on top of the greens, then place the remaining slices of bread on top.
Cut the sandwiches in half diagonally and serve immediately.
Notes
Serve with a side of colorfully sliced vegetables or a light salad for added freshness.