Trim the artichoke - with a strong, sharp knife slice off the stem so the flower sits flat (you can save it for another use) and cut off the top 2-3cm. Use scissors to cut the ends off the pointy leaves. Rub the whole thing with the olive oil/lemon mixture, paying particular attention to the cut sides. Drizzle the remaining mixture over the top.
Roast in a 200c oven for about 75 minutes. Serve.
Butter/Oil
Just before you are ready to eat the artichoke, melt the butter in a small saucepan, then add the lemon juice. You may need a little more or a little less than the recipe lists, depending on your preference - I like things tart. Season to taste and transfer to a small dipping pot.
Notes
To make this dish vegan, use olive oil instead of butter for the dipping sauce.