Fill up a medium bowl with water and add the lemon juice.
Clean the artichokes: Cut off top 1/3 of the artichoke, including spiny tops. Remove the outer leaves by peeling them back until they snap. Using a paring knife, peel stem to remove fibrous outer layer. Remove any remaining tough, fibrous, dark green parts on the outside of the artichoke heart. With a spoon, scoop out the chokes and clean the inside of the artichoke. Immediately put artichokes in bowl of water with lemon.
Preheat the oven to 400 degrees F (200C).
In a small (1-quart [948-mL]) saucepan, bring to a boil the vegetable stock, oil, garlic, coriander, a pinch of salt and a grinding of black pepper.
Add the artichokes, lower the heat to medium, and cook for 10 minutes.
Drain and artichokes and save the cooking liquid. When the artichokes are cool enough to handle, dice them.
In a medium bowl, combine the chèvre, artichokes, celery, and 3-4 tablespoons of the artichoke cooking liquid. Stir well.
Gently open the zucchini flowers and using a spoon, fill each flower with as much of the goat cheese (chèvre) mix as it will hold. Arrange the flowers in an oven-safe dish, drizzle with 3 tablespoons the cooking liquid and bake for 5-7 minutes.
Cool and refrigerate for 2-4 hours.
To serve, place two zucchini flowers on each plate. Add some mesclun and dress it with a little of the artichoke cooking liquid and ¼ to ½ tablespoon balsamic vinegar.
Garnish with the tomato and serve.