In a small bowl, mash avocado with garlic powder, and lemon juice. Season to taste with salt and pepper.
On one slice of bread, spread avocado mixture. Top with feta, tomato slices, cucumber slices, red onion, sprouts, and arugula.
One other slice of bread, spread Dijon and place this slice, Dijon-side down, onto the sandwich. Slice in half and serve immediately.
Notes
Storage: Store any leftovers wrapped tightly in plastic wrap. It will last for 24 hours, but the sandwich may soften, so enjoy immediately for best results.