Get out a skillet and put over low heat. Add the oil, green onion, and garlic and cook just until onion softens slightly and garlic is fragrant; remove from heat and set aside.
In a large bowl, mix together the fish, yuca, bread crumbs, mustard, cumin, red pepper flakes, salt, black pepper, and cilantro; blend with a fork until combined, then add the egg and onion/garlic, and mix until incorporated.
Form into patties and set aside while you prepare a skillet.
Get out a large heavy skillet and put over medium-high heat. Add cooking oil enough to come up to 1/8 inch in skillet. When hot, carefully add the patties a few at a time and cook until browned on both sides. This should take only a few minutes on each side.
Remove and let cool slightly before serving. May serve alongside a salad or in a bun like you would a burger. Top with guacamole, salsa, or sour cream if you wish.