In a mini food processor, add carrots and celeries. Process until very finely chopped. If you don’t have a food processor, you can cut them with a knife. You should have about 1 cup of minced vegetables.
Preheat the oven to 300˚F. Adjust the oven rack to one level below the middle.
In a medium Dutch oven over medium heat, add butter and olive oil. Once butter is melted, add onion. Saute until soften, about 2 minutes. Add carrots and celeries, cook for 2 minutes. Add garlic and saute for 1 more minute. Season with salt and pepper.
Add beef and cook until no longer pink, about 5 minutes, stirring and breaking apart with a spatula. Add white wine. Scrape off any brown bites at the bottom of the pan if there is any. Reduce for 2 minutes. Add tomato paste, milk, crushed tomatoes, and nutmeg, mix well. Add bay leaf and parmesan rind. Bring to a simmer and cook for 3 minutes. Season with salt and pepper. Cover with a lid.
Transfer to oven and cook for 1 hour (up to 1½ hours). About half way, take out of the oven and use a large spoon to remove some of the fat that float to the top of the sauce. Taste and adjust with more salt and pepper. Stir and bring back to the oven.
When bolognese is almost ready, bring a large pot of water (salted) to a boil over high heat. Cook the tagliatelle according to the package instructions until tender but still firm to bite, stirring occasionally. Reserve 1 cup of pasta water. Drain pasta. Set aside.
5 minutes before pasta is ready, remove the dutch oven from oven and set over the stove. Remove more fat if desired. Remove bay leaf and Parmesan rind as well. Scrape down side of the pot to remove the brown spots (flavors) and stir into sauce. Taste and adjust the seasoning. Add the pasta, a little bit of pasta water, and ¼ cup grated Parmesan. Using a tong, mix everything. Add more pasta water if needed. Transfer to serving plates and top with more Bolognese sauce. Grate extra Parmesan on top and serve hot.