Heat a medium Dutch oven over medium heat. Add the butter and oil and warm until the butter is melted. Add the carrots, celery, onions and 1/2 teaspoon salt. Cook, stirring often, until the vegetables are soft but have no color, about 6 minutes. Add the beef and lamb and cook, breaking apart the meat with a wooden spoon, until the meat is cooked through and no longer pink, about 5 minutes. Stir the tomato paste, chili paste, red pepper flakes and garlic into the meat mixture. Cook the tomato paste, stirring often, about 2 minutes. Add the milk. Bring to a simmer and cook, stirring occasionally, until the milk is almost entirely evaporated, about 20 minutes.
Add the wine, bay leaf, Parmesan rind, tomatoes and 1/2 teaspoon salt. Bring to a simmer and reduce the heat to low to just maintain a gentle simmer. Simmer the sauce, stirring occasionally to prevent sticking, 1 1/2 to 2 hours. Skim the oil from the surface.
Spoon the sauce over the Creamy Polenta with Spinach, reserving any extra to serve on the side. Serve with the Parmesan sprinkled over the top and a drizzle of olive oil.
Creamy Polenta with Spinach:
In a Dutch oven, bring the chicken broth, oil, garlic and 1 1/2 cups water to a boil over medium-high heat. Reduce the heat to medium low and whisk in the polenta. Season with 1/2 teaspoon salt and cook, stirring occasionally with a wooden spoon, about 10 minutes. Stir in the Parmesan, mascarpone, pecorino and butter. Cook, stirring often, about 5 minutes. Fold in the spinach and cook until wilted, about 5 minutes longer. Add 1/4 cup water if the mixture gets too thick.
Pour the polenta onto a large board or platter. Serve with grated Parmesan over the top.