Combine the artichoke hearts in a bowl with the mayonnaise, spinach, Parmesan, yogurt, garlic powder and lemon zest and juice. Taste the mixture and season if necessary.
Cut vertical lines in the boule 1 inch apart, cutting halfway deep. Turn the bread 90 degrees so that the lines are horizontal, and repeat the previous step so you end up with 1-inch cubes still attached. Spread the cubes slightly apart and evenly spoon the artichoke-spinach mixture into the crevices. Put the boule on a baking sheet and top with the Gruyere.
Bake until the cheese is bubbling, 10 to 15 minutes. Pull off the cubes and enjoy.