Heat the olive oil in a frypan and add the chopped onion. Saute for a few minutes until translucent but not brown. Add the onion bulbs, garlic bulbs, carrots, potatoes artichokes and stems. Just cover with water and the lemon juice. Add a small piece of baking paper over the mixture, bring to the boil and cover with a lid. Reduce to a simmer for around 45 minutes.
Prepare the sauce: in a separate bowl, add the lemon juice and combine with the cornflour. In another separate bowl, beat the egg and then slowly whisk in the the lemon juice mixture until frothy. Next, slowly add the broth from the pot whisking continuously (about 2 tea cups worth). Finally, add the sauce back into the pot covering the artichokes and vegetables Return to a very very low heat and swirl the sauce by shaking the pot from the handles. Do not stir or allow the sauce to boil. The sauce will slowly thicken.