In a small skillet over low heat, melt butter. Add onions and garlic. Cook for 2 minutes.
Boil water in a pot and cook the frozen yuca for about 30 minutes or until tender all the way through. Drain. Transfer to a large bowl and mash.
Separate 1 cup of the mashed yuca and place in a medium bowl. In that same medium bowl place the fish, cooked onions and garlic, bread crumbs, cilantro, mustard, cumin, red pepper flakes, salt and pepper.
In a separate bowl, whisk together the cream and eggs until well blended. Add to the fish mixture and mix well.
Form the mixture into round cakes. In a large sauté pan over medium-high heat fry the cakes in the oil, turning once, until lightly browned, about 3 minutes per side. Serve with your favorite sauce or just limes.