Wash artichokes by plunging them up and down in cold water. Cut off stem end of each artichoke; remove bottom leaves. Trim about 1/2 inch from the top of each artichoke; trim away about one-fourth of each leaf (tip). Rub the top of each artichoke and trimmed edges of leaves with cut lemon to prevent discoloration.
Arrange the artichokes, stem ends down, in a large Dutch oven; cover with water, and add the lemon juice. Bring to a boil cover, reduce heat, and simmer for 30 minutes or until a leaf near the center of each artichoke pulls out easily. Drain and set aside.
Combine vegetable stock and oil in a nonstick skillet; place over medium heat until hot. Add onion and garlic; sauté 3 minutes. Add zucchini, tomato, parsley, and bell pepper; cook 13 minutes or until tender, stirring occasionally. Remove from heat; stir in salt and pepper.
Scrape out the fuzzy thistle center (choke) from each artichoke with a spoon, and discard. Spoon 1/2 cup vegetable mixture into center of each artichoke, and serve immediately.