Place a large pan of water on a high heat and bring to the boil then place the trimmed Brussels sprouts into the boiling water and cook for five minutes. When the sprouts are cooked, immediately drain off the water and run them under cold water to retain their colour and crunch.
When they have cooled, slice each sprout into quarters (this is optional but doing so helps the sprouts to carry more flavour).
When you are ready to serve, place a large frying pan on a high heat and add the olive oil and butter. When the butter has melted, add the chopped garlic and chilli and give the pan a little toss, being careful not to brown the mixture as this will taint the flavour of the chilli and garlic.
Add the quartered sprouts, followed by the lemon zest and juice and give the pan another toss to ensure all sprouts are well coated. Serve immediately; the sprouts should still be a vibrant green colour and be sweet and crunchy with a good citrus and chilli kick.