In a food processor, pulse the onion, celery, and carrot until finely chopped.
Heat the olive oil and butter in a large Dutch oven over medium heat. Add the vegetables and cook, stirring occasionally, until they are soft and golden, about 5-7 minutes.
Meanwhile, add the tomatoes and their juice to the food processor and pulse 5-7 times until smooth. Transfer them to a bowl and set aside.
Wipe out the food processor, then add the pancetta to the bowl. Pulse until the pancetta is a coarse paste. Set aside.
Add the ground beef, pork, and pancetta to the Dutch oven, along with the red pepper flakes. Break the meat apart as it cooks, just until slightly browned, so the meat isn't raw.
Add the tomato paste and cook for 10 minutes, stirring occasionally, until the paste begins to brown.
Add the wine and cook until it is almost all absorbed, 10 minutes, stirring to scrape up any browned bits.
Add the milk and cook, stirring occasionally, until it has evaporated, which takes 30 minutes.
Add the blended tomatoes, beef broth, bay leaves, kosher salt, and freshly cracked black pepper to taste. Bring to a simmer, then reduce the heat to the lowest setting so it cooks with barely a bubble breaking the surface occasionally.
Cook uncovered for 2½ to 3 hours, occasionally spooning off the fat that collects on top, until the meat is tender and the sauce has reduced and thickened to become rich and dark in color.
Serve the sauce over rigatoni or wide pasta noodles such as pappardelle, tagliatelle, or fettuccine.
Serve topped with freshly grated Parmesan cheese. Enjoy.