General tips
Halve them lengthwise before bagging. Whole brussels sprouts cook unevenly because the outer leaves are exposed and the dense core takes longer. Halving exposes the core to the heat directly and creates a flat surface for the post-bath sear.
Bag with butter and bacon fat for max flavor. Brussels sprouts love rendered pork fat, and the closed bag locks the flavor against the cut surfaces. A few strips of bacon, rendered to crispy and crumbled, plus a tablespoon of butter, is the right move.
Cook at 183°F for 1 h. The bath softens the dense interior to tender without going to mush, and the cut surfaces are ready to sear into a deeply caramelized crust. Anything shorter and the centers are still slightly raw.
Sear cut-side down in a screaming-hot cast iron skillet, no oil needed (the bacon fat from the bag is enough). 2 min without moving them, then a flip and 30 sec on the rounded side. The cut sides should be nearly black with caramelization.
Anti-tip: don't trim too much off the stem end. Brussels sprouts hold together by that stem - over-trimming causes the leaves to fall off as they cook. A small flat cut is enough; leave the rest of the stem intact for structure.
Tender tips
Score an X in the stem end for faster, more even cooking.
Add bacon fat or butter to the bag for rich flavor.
Quick sear in hot pan after cooking adds caramelized bits.
Very Tender tips
Reserve cooking liquid for soup base - it's sweet and flavorful.
Add cream or mascarpone before puréeing for silky texture.
Top purée with crispy shallots or pancetta for contrast.