Few vegetables pair with pork better than fennel. Here we've rubbed boneless tenderloins with fennel pollen, given them a quick sear in a cast-iron pan, then finished the meat in a hot oven. The sauce is a luscious purée of fresh fennel enriched with mascarpone and parmesan cheeses. To finish the presentation, nap the meat with the sauce, then sprinkle with a gremolata made with panko, lemon zest, and herbs. If you're short on time, skip the gremolata; the meal will still be delicious.
Dry the pork and then rub with the olive oil. Place the meat in a large roasting or baking pan and season with salt, pepper, and fennel pollen. Cover tightly with plastic wrap and refrigerate. This step may be done a day in advance of serving.
Combine the fennel and chicken stock in a saucepan and bring to the start of a boil over high heat. Reduce the heat to low, cover the pan, and cook until the fennel is very tender, 20 to 30 minutes. Note that baby fennel will cook much more quickly than older, larger bulbs. Drain and transfer the fennel to a food processor. Purée until smooth. For a silky sauce, pass the fennel through a fine mesh sieve, discarding any solids.
Transfer the purée to a clean saucepan and add parmesan, mascarpone, and fennel pollen. Reheat over low heat, stirring until the cheeses melt. Season with salt and pepper to taste.
Meanwhile, make the gremolata by combining the panko, parsley, fennel pollen, lemon zest, melted butter, and salt in a small bowl, stirring to blend. Set aside.
Position a rack in the middle of the oven and preheat to 375 degrees F.
Heat a large oven-proof skillet, preferably cast iron, over medium-high heat. When hot, add the pork and sear it on all sides. Don't crowd the pan; if necessary, do it in two batches. Arrange the pork in the skillet and transfer to the oven. Roast until the meat reaches the desired degree of doneness, 8 to 20 minutes, depending on how you eat your pork.
Remove the skillet from the oven and transfer the pork to a cutting board. Let the meat rest 10 minutes before cutting it into thick slices. Arrange the pork on a warm platter and nap the meat with the sauce. Sprinkle the gremolata over the pork and serve immediately.