Bring a large saucepan of water to the boil. Add the lemon halves. Cut 2cm from the tops of the artichoke leaves. Trim artichoke stems and peel away 2 layers of the outer leaves. Cut each artichoke in half and remove the choke (the furry part) near the base of the leaves. Immediately place artichoke in the saucepan of boiling water - artichokes will discolour if not cooked immediately. Cook for 20 minutes until tender when pierced with a skewer. Drain and discard lemon. Set aside for 15 minutes to cool. Place artichoke into the food processor. Add the ricotta, sour cream, spring onions, parsley, garlic, lemon juice and Tabasco, and process until well combined and smooth. Taste and season with salt.
Serve artichoke dip with crusty fresh bread or pita bread.