Season the pork with salt and pepper, to taste, the fennel seed, and the red pepper flakes. Dredge the pork in the flour.
Heat a large skillet with 3 tablespoons of the extra-virgin olive oil, over medium-low heat. When the oil ripples, add the pork and brown it for 5 to 6 minutes, turning once. Remove the pork to a plate. Add the remaining tablespoon of extra-virgin olive oil, then add the sliced fennel, onions, garlic, and thyme. Season with salt and pepper, to taste, and cook until softened, 8 to 10 minutes. Add the tomato paste and cook for 1 to 2 minutes more, then stir in the wine and scrape up the drippings. Stir in the citrus zest, juice, stock, and parsley. Reduce the heat, and simmer for 5 minutes, then add the pork and simmer for 3 to 4 minutes more. Turn off the heat and sprinkle with the chopped fennel fronds. Serve the pork and sauce in shallow bowls with crusty ciabatta for mopping.