Fill a large saucepan with enough cold water to cover the artichokes and stir in 1 tablespoon salt, 3 tablespoons lemon juice and 1 tablespoon flour.
Cut off and discard the stems of the artichokes, bend back the outer leaves until they snap off close to the base and trim the tops, dropping the artichokes as they are trimmed into the water. Simmer the artichokes, weighted with a heatproof dish or pie plate to keep them immersed, for 12 minutes, or until the bottoms are tender when pierced with a knife, and drain them.
Cut each artichoke in half and reserve. In a large skillet heat 2 tablespoons of olive oil over moderate heat and cook the garlic and red pepper flakes for 1 minute.
Add the shrimp and cook for 2 to 3 minutes until just pink. Transfer shrimp to a side plate.
Add the remaining oil to the skillet and heat until hot. Add the artichoke halves. Season with salt and pepper and stir in remaining lemon juice. Toss to combine and heat until the artichokes are just warm.
Return the shrimp to the skillet. Sprinkle in oregano and toss to coat. Adjust seasoning, if necessary. Serve warm topped with crumbled feta cheese.