How Many Leaves?
Just like many greens such as spinach, it takes a lot of leaves to feed the family. However much you would like, start with a little more than double your desired amount. For example if you would like to end up with 1 Cup of cooked greens, begin with 2 1/2 (ish) Cup of leaves. This ratio will vary of course, just keep in mind that they shrink up a lot when cooked down.
After draining all water from the pot, place approx 1-2 Tbsp. of Olive Oil with 1 Tbsp of Butter, melted in pan over medium-high heat. Dump the leaves back into the dry pan and mix with wooden spoon. Aside, rough chop 2-3 Shallots. Wait to add shallots.
Boil / Blanch the Leaves
To begin, cut the leaves off of the stalk at the stem, and place in a colander to wash thoroughly. Then, cut the middle stem out of each leave, fold over and cut into 1 inch ribbons. Then, boil (or 'blanch') the leaves, placing them in room temperature water, bring them to a boil with a healthy pinch of kosher salt and keep the water at a rolling boil for 7-9 mins. This may seem excessive, but the leaves are quite thick, and I found this to be necessary for a tender bite. I like to use a pot that has a steamer basket in it, so that I can drain the leaves easily. After this time, pull the leaves out and place aside on a towel, still in the basket. Drain all the water in the pot.
Prepare to Sautee, Sautee
Your Patience is Rewarded
Ok...now if you prefer your leaves to stay green like spinach, you can stop now and get a plate, BUT what I like to do is cook the mixture waaaay waaaaay down, until the beef stock and vinegar cooks out and the Brussel sprout leaves get brown and crispy. This definitely takes some time, but the caramelized shallots and leaves are (OMGoodness!) soooo tasty!