Start by steaming the potatoes. While the potatoes are steaming, warm a pan to cook the onions. Cook onions for just a few minutes, stir in garlic for a minute, then remove and place in a larger bowl. Don't clean the pan, you will cook the croquettes in it.
Once the potatoes have been steaming for roughly 10 minutes, toss in the tilapia and cook for another 5-8 minutes, until it's mostly cooked through and the potatoes are tender and easily pierced with a fork.
Add the potato & fish mixture to the onion bowl. Allow to cool for a minute, then stir in the egg, parsley, 1/4 cup of bread crumbs, juice from the lemon, a little pepper, and a good amount of salt. Form into little patties, you should get roughly 10.
Place the remaining 1/4 cup of breadcrumbs in a dish with a little salt & pepper. Lightly coat each patty.
Heat some oil (about a heaping tbsp) in the pan. Cook croquettes for a few minutes on each side. You will need to add more oil for each new round. Serve with the yogurt sauce and fresh lemon wedges