For the cabbage filling: Have ready a shallow 8- by 12-inch baking dish, or one of similar size.
Place the pancetta in a large sauté pan or skillet over medium-high heat. Cook for 3 to 4 minutes, adjusting the heat as necessary, until it starts to release its fat. Add the diced onion and cook 3 to 4 minutes until it has softened. Add the cabbage and season with salt and pepper to taste, using tongs to turn it over. Add 2 tablespoons of the chicken stock or broth and cook for 8 to 10 minutes, stirring often, until the cabbage has softened and almost all the liquid has evaporated. Move the cabbage to the sides of the pan to create a cleared center area. There, add the oil and the flour, stirring to combine, and then mix with the cabbage. Add the remaining warmed chicken stock or broth and mix well. The broth should thicken in a minute or two.
Remove the pan from the heat and add the cannellini beans, stirring to combine. Taste and adjust seasonings as needed. Transfer the mixture to the baking dish. Set aside.
For the mashed potato topping: Place the potatoes in a pot of lightly salted cold water. Bring to a boil over medium-high heat and cook the potatoes about 15 to 20 minutes, until easily pierced with a fork. Drain and mash to the desired consistency, adding the warm milk and butter; season with salt and pepper to taste.
To assemble: Preheat the oven to 400 degrees.