The liver was frozen when I bought it, so I put it in the fridge to thaw overnight. Liver is easiest to cut when it's half-frozen, so don't worry about thawing it fully. Leave it a little frosty. If it comes to you not-frozen, pop it in the freezer for 15 minutes to firm up. You're going to want to slice it about 1/4 to 1/3 inch thick. (That's between 1/2 cm and just under 1 cm for you metric folks.)
How many does this serve? Well, that depends on the size of your liver, and your appetites. It's two to three servings if this is all you're eating for a big meal, four or more if this is one course or part of a bigger meal, with some cauliflower rice, roasted squash, or even topped with a poached egg. Eat until you feel kind of 80% full, and you're happy and satisfied, then stop. Follow your appetite, not my rules! Save your leftovers for pate, or just for yummy snacks later.
A couple spoons of ghee or other fat that tolerates high heat
3 onions, thinly sliced
Salt to taste
Fresh or dried thyme, a teaspoon or to taste
One fresh or frozen lamb liver
1/4 cup or more balsamic vinegar
Fresh baby spinach, washed and spun dry
Heat a big Dutch oven or other large pan with one spoon of the ghee. Add the onions, and stir around to coat thoroughly with the fat. Let cook on fairly low heat, stirring only very occasionally, until golden brown all over. If you keep stirring them, they won't caramelize. Season with salt and thyme. This can take 15 - 20 minutes. Be patient. The slower you do this step, the deeper the flavor.
Heat your cast iron skillet on fairly high heat. Slice lamb liver into nice thin slices, and sprinkle on all sides with sea salt. Add a spoon of ghee to the pan, and fry liver in batches, about two minutes on a side, until medium rare. Transfer to a dish to keep warm while you cook the rest of the liver.
Deglaze the pan with the balsamic vinegar, and make the reduction. Pour the vinegar into the hot pan. It should boil and reduce right away. When it's down to half its volume, pour some of it in the onions and stir to combine.