In a cold, large cast iron skillet set over medium heat, cook the bacon until nice and crispy.
While the bacon is cooking, rinse the beef liver slices under running water and pat them dry.
In a shallow bowl or plate, combine the arrowroot flour, salt and pepper. Mix thoroughly with a whisk until very well combined.
Dredge the liver slices in the arrowroot mixture and shake well to remove any excess. Set aside in a plate until bacon is done cooking.
When bacon is nice and crispy, move it to a plate with a slotted spoon, and set aside. Pour the bacon fat into a small bowl but leave about 2 tablespoons in the pan. Put the skillet back over high heat.
When the pan is really nice and hot, add the liver slices and sear for about 45 seconds to a minute per side, just long enough for them to get a beautiful dark brown and crispy exterior.
Remove the liver to a plate, cover loosely to keep it warm while you work on the onion compote. Put your pan back over the heat source and lower heat to medium-high; add about half the remaining bacon fat and throw the sliced onions in.
Let the onions caramelize for about 10 minutes, stirring occasionally. When the onions are a nice golden colour, add the remaining bacon fat and garlic. Continue cooking for 2-3 minutes.
Add vinegar and water and cook for another minute or so, until liquid is completely evaporated.
Stir in fresh rosemary, turn off the heat and place liver slices on top of the onion compote. Cover loosely and let sit for about 5 minutes just to warm up the liver and allow all the flavours to mingle happily.