Season the pork with fennel, salt, and pepper, and dredge in flour. Heat a few tablespoons extra-virgin olive oil, a couple turns of the pan, in a Dutch oven over medium-high heat. Add the pork and brown the meat. Do this in 2 batches if your pot size demands it. Use a little extra oil if necessary for a second batch. Remove the browned meat to a plate and add the butter to the pan. Reduce the heat a bit and add the onions, apples, bay leaves, and sage. Season with salt and pepper and cover the pan to sweat the onions for 12 to 15 minutes, stirring occasionally. Uncover the pot and raise the heat a bit. When the onions start to brown, add the calvados to the pot. Add the stock, slide the pork back in, and simmer to cook through a few minutes more. Serve from the hot pot.