Preheat the oven (200C fan/220C conventional) and place a shelf in the lower part of the oven.
Give the artichokes a rinse and pat dry. Slice the stems with a sharp knife, so that the artichokes sit upright. Trim the top end of the leaves with a sharp knife.
Remove the outer petals and trim the ends of the rest with kitchen scissors. Splay the petals out and, with a small knife, cut around the heart and remove the 'choke', a mass of fine needle-like hair.
Mix the olive oil and lemon juice in a little bowl.
Place each of the artichokes inside a roasting bag and drizzle with the lemon/oil. Scatter the garlic, so that it goes between the petals, and season with salt and pepper. Tie the roasting bags and cook for an hour.
Carefully cut into the roasting bags – be careful as hot steam will escape – and remove the artichokes. They are ready when the petals are easy to pull off.
Serve with some garlic aioli or simply on their own. Only the soft bottom part of each petal is edible. Dip in your aioli and pull at the soft fleshy bit with your teeth.