Combine ingredients: Add the avocado, white beans, red onion, avocado oil, sriracha, and nutritional yeast to a medium bowl. Grate the garlic into the bowl using a microplane or fine grater. Add the lime zest, and juice.
Mash: Use a potato masher to mash the ingredients until the desired consistency. Season with salt and pepper.
Make the sandwich: Place the leafy greens onto a slice of bread. Top with the desired amount of the avocado-white bean mash. Add the cucumber slices, if using. Place another slice of bread on top.