In a large saucepan, heat oil over medium-high heat. Add veal and cook, stirring frequently with a wooden spoon, until meat is broken into small bits and browned, about 5 minutes. Add onion, 1/4 teaspoon salt and generous pinch pepper. Reduce heat to medium and cook, stirring occasionally, until onion is softened, 5 to 7 minutes. Stir in 11/4 cups vegetable broth. Very gently simmer until broth is reduced to about 1/4 cup, about 45 minutes.

Meanwhile, fill a large bowl halfway with cold water and several ice cubes; add lemon juice.

Cut off top 1 inch of 1 artichoke with a serrated knife. Bend outer leaves backward until they snap off close to base, then discard several more layers of leaves in same manner until you reach pale yellow leaves with pale green tips. Cut off green tips. Trim dark green fibrous parts from base, sides and stem of artichoke with a small sharp knife, then cut artichoke lengthwise into 8 wedges; cut out purple leaves and fuzzy choke. Drop wedges into acidulated water. Repeat with remaining artichokes.

Drain and pat dry artichokes, then add to saucepan with veal. Cover and simmer until artichokes are tender, 10 to 12 minutes. Transfer mixture to a 1 1/2-quart baking dish.

Heat oven to broil.

In a medium saucepan, bring remaining 2 cups broth just to boil then remove from heat. In a large saucepan, heat butter over low heat until melted, then add flour; cook, stirring constantly with a wooden spoon, 2 minutes. Whisk in hot broth and pinch salt, increase heat to medium-high and cook, stirring constantly, until sauce thickens, about 1 minute. Pour sauce over artichoke mixture, stirring gently to incorporate. Broil gratin until top is lightly browned, 3 to 4 minutes.