With kitchen shears trim sharp points from artichoke leaves.
Trim stems; remove loose outer leaves.
Cut off 1 inch from tops.
Brush cut edges with lemon juice.
In a large covered saucepan or Dutch oven, bring artichokes and salted water to a boil; reduce heat to low.
Simmer until a leaf pulls out easily, 20 to 30 minutes.
Drain upside down.
Preheat oven to 450 F.
Cut cooled artichokes lengthwise into halves.
Remove fuzzy chokes and hearts.
Finely chop hearts; discard chokes.
In a medium skillet cook mushrooms, artichoke hearts, squash, onions and garlic in oil for 3 minutes, stirring frequendy.
Stir in tuna.
Place artichoke halves, cut sides up, in a lightly oiled baking dish.
Mound tuna mixture in centers of artichokes.
In a small bowl stir together remaining ingredients; sprinkle over tuna mixture.
Bake 5 to 8 minutes or until cheese is melted and topping is golden.