Trim artichokes; snap off tough outer leaves. Cut off about top third of artichoke. Snip any leaf tips remaining below the cut with scissors. Trim stem so artichoke will stand upright. Trim base. Spread leaves open to gain easier access to choke. Using a melon baller, scoop out choke. Squeeze some lemon juice directly onto heart; squeeze juice from lemons into bowl of ice water; add lemon halves and artichoke while preparing the rest.
In a 12-inch saute pan, cook onions and garlic in oil over medium heat, until tender and lightly caramelized, about 1 hour; let cool. Take two-thirds of onions, and coarsely chop; place in a bowl, and add breadcrumbs, olives, parsley, anchovies, and thyme. Season with salt and pepper; mix well.
Heat oven to 375 degrees. Drain artichokes upside down on a paper towel; gently open leaves. Fill centers of artichokes with mixture; place small amounts in crevices of leaves. Place a string around outside of artichoke; tie snugly. Repeat with remaining artichokes.
Strew remaining onions on bottom of large Dutch oven. Fit in artichokes; add stock to bottom of pan. Sprinkle each artichoke with Parmesan. Cover; bake 30 minutes. Uncover; bake until golden and crusty on top and hearts are tender when pierced, about 10 minutes.