Adjust oven rack to lower-middle position and preheat oven to 300°F.
Place chicken stock in a bowl. Sprinkle gelatin in to dissolve. Set aside.
Heat olive oil and butter in a large Dutch oven over high heat until shimmering. Add the onion, celery, carrots and garlic. Cook until the vegetables are translucent but not browned, 5-10 minutes. Add the meats. Increase the heat to high, and brown the meat stirring frequently for 15- 25 minutes, until the meat is browned.
Add tomato paste and cook, stirring constantly, until rust-colored and fragrant, about 3-5 minutes.
Stir in wine and scrape bottom of pan with wooden spoon to loosen any food that may have stuck to the bottom of the pan and bring to a simmer for 5 minutes. Add sprigs of fresh thyme, marinara sauce, milk and broth mixture and return to a simmer.
Transfer uncovered pot to oven. Cook, stirring and scraping down the sides of pot occasionally, for 2 hours. Some ovens run hotter and this may cause the sauce to dry. If so, add a splash of stock or water, if necessary to keep the sauce from drying out.
Remove from oven. Using a ladle, feel free to skim off all the fat that may have accumulated on the top.
Add the heavy cream and basil and bring to a simmer. Season with salt and pepper to taste. I like to take two garlic cloves and press them through a garlic press or grate on a rasp grater (Microplane) and add them to sauce. I find it really bumps up the flavor. This step is optional.
If you prefer your Bolognese a little more tomatoey, feel free to add another cup of your favorite marina sauce (I recommend Rao’s marinara sauce).
Serve with pappardelle or tagliatelle pasta and top with Parmesan cheese.