Whisk together first 4 ingredients until blended. Gradually whisk in 1/4 cup olive oil. Stir in onion, oregano, and fennel. Set aside.
Bring 3 cups water to a boil; add shrimp and cook 3 minutes or until shrimp turn pink. Drain and rinse with cold water. Peel shrimp, devein, if desired. Cut shrimp in half lengthwise. Set aside.
Cut off stem end so that artichoke will sit upright; place in a small saucepan. Squeeze lemon juice over artichoke. Add water to cover, salt, and 3 tablespoons olive oil; bring to a boil over medium heat. Cover; reduce heat, and simmer 20 to 30 minutes or until tender. Drain and cool.
Peel away leaves. Using a spoon, remove and discard thistle (choke) from the heart. Dice heart; add to vinaigrette.
Spoon vinaigrette on artichoke leaves; top with shrimp. Garnish, if desired.