Blanch Brussels Sprouts: Start some water boiling and add a few pinches of salt. Rinse and halve your sprouts. When the water comes to a rolling boil, add sprouts and set a timer for 5 minutes. Do not overcook!!! Boil exactly 5 minutes, rinse with cold water, strain and set aside.
In the meantime, chop your onions or leeks and nuts. Slice bacon if you are using it into half inch slices.
Heat a pan on medium heat and add bacon slices. Allow bacon to cook about 4-5 minutes, until fat starts to render in the pan. Add the nuts and stir. If you are using onions, add those too (wait if you are using leeks.)
Cook nuts and bacon until the bacon is almost done, then add butter. You can add leeks at this point or skip this step and add Brussel sprouts directly. When leeks just begin to soften (about 1 minute), add Brussels sprouts, sea salt and pepper.
Stir sprouts and turn most of them so their cut faces are resting against the pan. Use tongs for this. After about 2 minutes, stir the sprouts and sprinkle on oregano. Continue to cook, stirring every 2 minutes or so until the faces of the sprouts are all browned and onions begin to caramelize, 8-10 minutes. In the last 3-4 minutes, add vinegar (or lemon). This step is essential to cut any last bit of bitterness remaining in the sprouts. Use the taste test to determine precise cooking time (depending on the size of the sprouts.)
