Place avocado, olive oil and juice and zest of lime in a bowl and use a fork to mash until smooth.
Add white beans to the bowl and use a fork to mash into the avocados. Stir in the red onions and season with garlic powder, salt and pepper.
Place arugula on top of two slices of the whole grain bread. Divide the mixture on top and then add the other slices of bread. Slice the sandwiches in half and enjoy immediately.
Notes
Meal prep: Prep this sandwich ahead of time and make it an hour or two in advance before eating.