In a small bowl, mix the cornstarch in ¾ cup of the milk and set aside.
Line a baking sheet tray with at least a one-inch edge with parchment paper or heavy duty aluminum foil.
In a saucepan, warm the milk with oregano, parsley and zest of lime and set aside or keep on low heat until needed.
Heat a medium sized pot over medium-low heat, add the butter and olive oil and sauté the onion and garlic until soft and translucent. Add cod and lightly sauté 1-2 minutes.
Add the flour to the pot and mix to form a roux. Stir constantly until the roux starts to color. Pour in the warm milk in a slow, steady stream, mixing constantly with a whisk, until all milk is added and mixture is completely smooth. Slowly add the cornstarch mixture. Mix well and cook over low heat for 5 minutes, until very thick. Pour onto the baking sheet tray and spread it out evenly. Let cool for one hour, uncovered.
Fill a pastry bag fitted with a 1-inch tip or plastic gallon bag with the cooled mixture and cut off the tip of the plastic bag to make a 1-inch hole. Prepare your second sheet tray with about 1/4-inch layer of flour. Pipe a long tube of mixture onto the tray and continue until you have piped all of the mixture onto the tray and you have about 5-6 tubes. Refrigerate until firm, about 2 hours.
Cut each tube into 2-3 inch logs and dredge well in flour on all sides. In a shallow dish, beat the eggs with a little water and spread breadcrumbs on a cookie sheet or shallow dish. Dredge each croquette in the egg mixture, then the breadcrumbs to coat completely and place on a clean tray.
Fill a heavy pot with frying oil, 2-3 inches deep. Heat to 350° F and fry croquetas in batches, turning them gently until golden brown on each side. Transfer to an ovenproof platter lined with paper towels and keep warm in a low oven until all croquetas are done.
Serve warm with alioli sauce and lemon wedges.