Prepare the fava beans: Rinse the fava beans in a colander and pour them into a saucepan. Add coarsely chopped onion, water, and the Knorr vegetable cube. Transfer to low heat for about 40 minutes, stirring at regular intervals to prevent the fava beans from sticking.
Make the skordalia: Transfer the cooked fava beans to a food processor. Add sliced garlic, vinegar, olive oil, and pepper. Beat until smooth. Taste and add salt as needed.
Prepare the cod: Mix cornflour, flour, and a little salt in a baking pan. Dip the cod first in water, then in the flour mixture, back in the water, and again in the flour mixture. Fry in sunflower oil over high heat in two batches for 4 to 5 minutes or until crispy.
Serve: Remove the cod with a slotted spoon onto a baking sheet with absorbent paper. Transfer to a platter and serve with lemon slices, garlic fava bean skordalia, and chopped parsley.