Adjust an oven rack to the lower-middle position.
Preheat the oven to 300 degrees F.
In a medium bowl, add the stock and the gelatin.
Transfer the tomatoes to a blender and purée until smooth.
Place the chicken livers in a cup that just fits the head of an immersion blender and purée until smooth.
In a large Dutch oven over high heat, heat the olive oil until it is shimmering.
Add the ground beef, the pork, the lamb, the salt, and the pepper to the Dutch oven and cook, stirring and breaking up the meat with a wooden spoon, until no longer pink, about 10 minutes.
While the beef cooks, in a large skillet over medium-high heat, add the butter and the pancetta and cook, stirring frequently, until the fat has mostly rendered out but neither the butter nor the pancetta have started to brown, about 8 minutes.
Take the beef mixture off the heat and stir in the puréed chicken livers.
Add the onion, the carrots, the celery, the garlic, the sage, and 1/2 of the parsley to the pancetta mixture and cook, while stirring and tossing, until the veggies are completely softened but not browned, about 5-8 minutes.
Add the veggie mixture to the beef mixture.
Return the Dutch oven over high-heat and cook, while stirring, until most of the liquid had evaporated from the pan, about 10 minutes.
Add the wine to the meat mixture and cook, while stirring, until the liquid is mostly evaporated.
Add the reserved stock, the puréed tomatoes, the milk, the bay leaves, the salt, and the pepper to the mixture and bring to a simmer.
Transfer the sauce to the oven, uncovered, and cook, stirring and scaping down the sides of the pot occasionally, until the liquid has almost completely reduced and the sauce is rich and thick underneath a heavy layer of fat, about 3-4 hours.
If after 4 hours it looks like the liquid or fat in the sauce has not yet separated and formed a thick layer, transfer it to the stovetop and finish cooking at a brisk simmer while stirring frequently.
Carefully skim most of the fat from the sauce, only leaving about 1 cup of fat.
Stir the heavy cream, the parmesan, the fish sauce, and the remaining parsley into the sauce.
Bring the sauce to a boil on the stovetop, stirring constantly to emulsify.
Season the Bolognese with the salt and the pepper.
Cook the pasta in a large, well-salted pot of boiling water until just barely al dente.
Drain, reserving 1/2 cup of the cooking liquid.
Return the pasta to the pot and add just enough of the Bolognese sauce to coat, along with some of the reserved cooking liquid.
Cook over high heat, tossing and stirring gently, until the sauce is thick and the pasta is coated, about 30 seconds.
Serve immediately.