Step 1Heat the oven to 400 degrees. Use the melted butter to generously butter 3 madeleine tins; set them aside.
Step 2Place the grated onion in a kitchen towel and wring out the excess liquid. Set aside.
Step 3Combine the eggs, cream, mustard, garlic, basil, salt, black pepper and cayenne. In a separate bowl, combine the flour and baking powder, then stir the flour mixture into the egg mixture. Add the zucchini, onion and cheese and mix very well.
Step 4Spoon the batter into the prepared tins, filling each indentation to the edges. Bake 25 minutes, or until puffed and golden brown (centers will still be moist). (This is the cooking time using traditional madeleine tins. For nonstick tins, bake for about 20 minutes; for silicone molds, bake for 28 minutes.)
Step 5Use a small offset spatula or knife to immediately loosen the madeleines and turn them out of the tins. Serve warm.