Combine the onions, celery, bell peppers, 1 teaspoon of the salt, and 1/4 teaspoon of the cayenne in a mixing bowl. Let sit for 1 hour at room temperature.

Sprinkle the butter slices evenly with 1 teaspoon of the salt and 1/4 teaspoon of the cayenne. Freeze for 30 minutes. Toss the butter with the vegetables.

Preheat the oven to 400º F.

Remove any extra fat around the opening of the cavity of the goose. Prick other fatty areas of the goose with a fork at intervals. Do not prick the breast. With the goose breast side up and the cavity facing you, make slits using a sharp pointed knife, inside the cavity between the rib bones and the breastbone. Do not puncture the breast skin. Using your fingers or the handle of a small spoon, insert 2 to 3 slices of butter and about 1 tablespoon of the vegetables into each hole. Season the outside of the goose with the remaining 1 teaspoon salt, the remaining 1/4 teaspoon cayenne, and the black pepper.

Place the goose in a rack set in a large roasting pan and roast for about 1 hour. Remove the goose from the pan and stuff it with the dressing. Reserve any excess dressing to garnish the serving platter. Return the goose to the oven and roast for about 45 minutes, or until drumsticks and thighs are easy to move.

FOR THE PORT GRAVY: Heat the oil in a saucepan over medium-high heat and brown the giblets and neck, 3 to 4 minutes. 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 for about 5 minutes. Add the wine, bring to a boil, and cook, stirring occasionally, for 5 to 6 minutes. Add the water and bring to a boil. Reduce the heat to medium and simmer for about 1 hour. Strain.

To serve, carve the goose and serve with rice dressing and the port gravy.