1. Preheat oven to 400 degrees F.
2. Combine 1/2 cup of the onions, celery, bell peppers, 1 teaspoon of the salt, and 1/4 teaspoon of the cayenne in a mixing bowl. Remove any excess fat around the opening of the cavity of the goose. Prick other fatty areas with a fork at intervals. Do not prick the breast. Stuff cavity of goose with vegetable mixture. Season the outside of the goose with the remaining 1 teaspoon salt, 1/4 teaspoon cayenne, and the black pepper. Place the goose in a large roasting pan and roast for about 1 hour and 45 minutes, or until the drumsticks and thighs are easy to remove.
3. Combine the remaining 1/2 cup onion, carrots, bay leaves, and port wine in a medium-sized sauce pan over medium heat and bring to a boil. Cook over medium heat until the mixture thickens and reduces to about 1/2 cup, about 30 minutes. Strain through a fine mesh strainer and let cool.
4. Using a pastry brush, glaze the goose with the port wine reduction, and return to the oven for 5 minutes to allow the glaze to caramelize and set. Remove from the oven and let rest for 20 minutes before carving. Serve with the Port Gravy, and Goose Fat-Roasted Potatoes.

Port Gravy:

1. In a large saucepan, heat the oil over medium-high heat. Add the giblets and neck, and brown, stirring, 3 to 4 minutes.
2. Add the onions, celery, carrots, salt, pepper, bay leaves, and thyme and cook, stirring often, for 3 to 4 minutes. Add the flour and stir to combine. Cook, stirring, for 5 minutes.
3. Add the wine, bring to a boil, and cook, stirring occasionally, for 5 minutes. Add the water and bring to a boil. Reduce the heat to medium and simmer for about 1 hour. Remove from the heat and strain into a clean container and keep warm until ready to serve. 

Goose Fat Roasted Potatoes:

1. Scrub potatoes and place in a large pot. Cover with cold water and add 1 teaspoon salt. Bring to a boil. Reduce the heat to medium-low and simmer until just fork tender, 10 to 15 minutes. Drain and cool under cold running water. When completely cooled, slice them in half lengthwise, and place in a bowl.
2. Preheat oven to 400 degrees F.
3. Season the potatoes with 1 teaspoon of salt and 1/2 teaspoon of black pepper. Place a 12-inch, non-stick, ovenproof saute pan or skillet over medium-high heat and add the goose fat.
4. Once the fat is hot, add the seasoned potatoes to the saute pan, cut side down. Sear on the stovetop for 1 minute, then place the pan in the oven. Roast for 8 to 10 minutes. Remove the pan from the stove, and add the garlic and chopped rosemary. Toss the potatoes in the pan to coat them and serve immediately garnished with the rosemary sprigs.