Peel the onions, cut the tip a bit, and hollow them with a spoon. Set the content aside.
Put them in a loaf pan, season with salt, pepper, and add some olive oil and butter. Put in the preheated oven to 400°F for about 30-45 minutes, or until the onions look cooked.
While the onions are in the oven, prepare the stuffing; thinly dice the center of 1 onion and sauté with olive oil in a hot pot. Once cooked, add the diced carrot and then bell pepper, stirring well.
Make room in the center of the pot and place the diced chicken breast, trying to brown all surfaces, then stir well and add the diced zucchini. Season with salt and pepper, add mustard and cream, and once the chicken is cooked, add parmesan cheese. Stir and remove from the heat.
For the sauce, put blueberries, red wine, water and a pinch of salt in a pot, cook over medium heat until it has reduced and acquired the desired consistency.
Once the onions are cooked, take out of the oven, stuff each one with the chicken mixture, filling them well, add fresh blueberries to the oven dish and put in the oven again until obtaining an oven gratin.
Serve the onions topped with blueberry sauce.