Heat a dash of oil in a large saute pan over medium heat. Add garlic and cook for a minute, then add spinach. Cook, stirring often, until spinach has reduced in size, about 3 minutes. Remove from pan and roughly chop spinach so that it's not too stringy.
In a large mixing bowl, combine spinach, mashed potatoes, parmesan, and mozzarella. If mixture is too moist, add a dash of flour. Roll into tablespoon-sized rounds.
Prepare a work station by whisking eggs in a bowl, placing breadcrumbs in a separate bowl, and placing a clean plate at the end. Working in batches and with a spoon, dip each ball into the egg, let excess drip off, roll in breadcrumbs, then set on the plate.
Heat oil over medium/high heat in a large pan. It's ready when a piece of panko thrown in instantly begins sizzling. Working in batches of 4 or 5, gently add the croquettes, rolling slowly, until the entire coat is brown and crisp (about 2 to 3 minutes each)*. Set finished croquettes on a few layers of paper towels or a wire rack until you've finished them all. Serve immediately**.
*If using leftover mashed potatoes, be sure your potatoes have a bit of butter and milk mixed in and were seasoned with salt and pepper!
**If it takes any longer, the cheese and potato will begin to melt and soften, leading to deformed croquettes. You want them to turn brown pretty quickly, so crank up the heat!
***These don't do well as leftovers, so try to eat them fresh!