Begin by preparing the brussels sprouts for cooking. Cut a cross in the base of each sprout—this helps them cook evenly and allows the heat to penetrate to the center. Bring a large pot of salted water to a rolling boil and carefully add the 2 lb (1 kg) of brussels sprouts.
Simmer them for 5–10 minutes until they are just tender when pierced with a fork. The timing depends on the size of your sprouts, so begin checking around the 5-minute mark to avoid overcooking them and turning them mushy. Once tender, drain them thoroughly in a colander and set aside.
While the sprouts cook, prepare the signature mustard-seed butter. Melt 3 tbsp of butter in a large saucepan over medium heat.
Once melted, add the 2 tsp of mustard seeds and immediately cover the pan with a lid. Reduce the heat to low and let the seeds cook for 1–2 minutes. You'll hear the mustard seeds popping as they heat—this is the desired effect and indicates they're releasing their aromatic oils.
Once the popping subsides and the butter has turned a light golden-brown color, remove the pan from heat. This step requires attention, as the butter can burn quickly if left unattended, which would ruin the delicate, nutty flavor.
Add the drained brussels sprouts to the pan with the mustard-seed butter and toss them gently but thoroughly to coat them evenly. Return the pan to low heat briefly to warm the sprouts through. Remove from heat and add the 1 tbsp of lemon juice, stirring gently to distribute it.
Season to taste with salt and black pepper, adjusting as needed to complement the mustard and lemon flavors. Serve immediately while the dish is still warm and the butter coats the sprouts beautifully.
