Cut the chicken fillets into 4-5 slices each. Peel the zucchini and cut them into thin slices. Finely chop the green onions and fennel into thin slices. Remove the stem and seeds from the bell pepper and cut it into thin strips. Heat 4-5 tablespoons of olive oil and sauté the chicken and vegetables over high heat until they brown. Season with salt and pepper and transfer them to a bowl.

Prepare the sauce: In 3 tablespoons of olive oil, sauté the onion and garlic. Once they soften, deglaze with the wine and add the mustard, lemon juice, lemon zest, and oregano or marjoram. Season with salt and let it cook for 1 minute.

Divide the chicken and vegetable mixture among 4 pieces of parchment paper and drizzle with the sauce. Carefully fold the edges of the parchment to prevent the sauce from leaking during cooking. For added security, you can wrap each package in aluminum foil. Place the packets in a shallow baking dish and bake in a preheated oven without fan at 200ºC (392ºF) for 30 minutes. Place one packet on each plate and open it with scissors just before serving.