Bring a large saucepan of water to a boil. Add a heaping Tablespoon of kosher salt and the halved Brussels sprouts. Set your timer for 6 minutes (4 minutes for small sprouts, 7 minutes for large). In the meantime, prepare 2 layers of paper towels on a large rimmed baking sheet. After the sprouts have been in the water for 6 minutes, either drain them in a colander or remove them from the water with a slotted spoon. Transfer them to the paper towels to drain very well.
Go put on an apron if you are not wearing one, so any splattering doesn’t mess up your outfit. Heat a large, heavy-bottomed skillet over medium heat. If you do not have a large skillet, divide the oil evenly amongst 2 medium skillets, or sear the sprouts in 2 batches, adding oil between batches as needed.
When the oil is warm but not smoking, add sprouts in one layer. Sprinkle generously with sea salt and freshly ground black pepper to taste.
Allow the sprouts to brown without burning and then turn to brown on the other side. Cook until crispy on both sides.
In a separate small skillet, heat 1 Tablespoon of olive oil and the butter. Add the garlic and crushed red pepper and cook for 1 minute until the garlic just starts to brown lightly on the edges. Stir in the lemon zest and juice and turn the heat off.
Transfer the Brussels sprouts to a serving bowl and pour the lemon-garlic mixture over and toss to coat.