Rinse the liver and place it in a bowl. Cover with the buttermilk, give a light stir and set aside for at least 2 hours.
After that, take the beef liver out of the buttermilk. Rinse again and pat dry with paper towels. Cut the liver in slices, I personally prefer small ones but you may leave them larger.
Melt together ½ tablespoon of oil and ½ tablespoon of butter.
Cook the garlic for about 1 minute until fragrant.
Add the liver slices and fry until no longer pink. The discarded juices need to lose their pink color too. This will take about 3-4 minutes on each side. Make sure you don’t overcook the liver or it will become hard.
Remove the liver from the pan but do not wipe out unless there are many burnt bits.
Melt the remaining oil and butter. Add the onion rings and cook for 3-4 minutes. Also bring in the red bell pepper. I like my food to have splashes of color and this is one reason I’ve added the bell pepper. The other reason is that it gives a lot of flavor to this dish. Cook for 3-4 minutes more.
Pour the white wine. Reduce the heat to low, add the beef liver slices and simmer for 5 more minutes until the alcohol evaporates.
Top with the fresh thyme, cover with a lid and leave it to rest for five minutes.
Serve immediately after that!