Put beef liver slices in a bowl and pour over milk. Leave for 20 minutes (or an hour or two in the refrigerator). Pour flour into another bowl and add salt, spices, and mix.
Remove the liver slices from the milk, pat dry and cut into 2-inch pieces. Roll each piece of liver in flour on all sides so that it is covered.
Transfer the liver pieces to a pan over medium high heat with already heated olive oil. Fry for 3 minutes on one side and 2 -3 minutes on the other side. This way the liver will remain pink inside and very soft. (timing depends on thickness)
The internal temperature for organ meat should be 160 F in the thickest part. Remember that proteins will rise another 5 degrees as they sit but much higher will make it dry out and be too firm.
Transfer the fried liver to a napkin to absorb excess oil. Melt the butter in a frying pan and fry the onion until soft and caramelized. Add sugar and fry the onion for another 3-4 minutes until the sugar has melted completely. Transfer the onion to a plate and place the liver pieces