Add 1 tablespoon oil to a large deep skillet over high heat. Once the oil is shimmering, add in the ground beef. Season to taste with salt and pepper. Brown the beef, crumbling and breaking it up as you stir. Drain off fat and add meat to a slow cooker. Put the pork or sausage into the same skillet you cooked the beef in. (If you add sausage you'll likely need to add a tablespoon of oil.) Brown the meat, crumbling as you go. Again, drain off any rendered fat and add meat to the cooker. (We separate browning both types of meat to keep from stewing the meat. This helps a lot in the final texture and flavor.)
Add last tablespoon of oil to the same skillet, reduce the heat to medium low, and add in the finely diced onion, carrot, and celery. Cook, stirring occasionally, for 3 minutes. Add in the garlic and continue to cook, stirring occasionally for an additional 5-7 minutes until vegetables are softened and straw colored (you don't want to brown the veggies; reduce the heat if they start browning).
While the vegetables are cooking, add the crushed tomatoes, tomato paste, Worcestershire sauce, bay leaves, thyme, basil, oregano, bouillon cubes (crumble them up), and water to the crockpot.
Return to the vegetable mix and turn up the heat to high. Pour in the beef stock or wine and bring to a boil. Scrape any browned bits from the skillet. Once boiling, reduce the heat and simmer over medium-low heat for 3 minutes or until reduced by half. Remove from heat and add to the crockpot. Stir everything together.
Cover the slow cooker, set to low, and cook for 5-6 hours. Stir occasionally if possible. Once finished taste and adjust salt and pepper to personal preference. Also taste for sourness -- if you use high-quality tomatoes it shouldn't be sour. On occasion, however, you might need to add a teaspoon or so of white sugar to balance out the acid.
Near the end of the cooking time, follow package directions to cook the spaghetti to al dente. Remove about 1 cup of the pasta water.
Two ways to serve: Either emulsify the pasta: (authentic way; see direction #9) or just spoon the sauce over the cooked noodles.
Garnish with fresh chopped parsley or basil and freshly grated Parmesan cheese as desired.
To EMULSIFY: Bring a large pot of salted water to boil. Follow package directions to cook the pasta but don't drain (yet). Scoop out a cup of pasta cooking water and set aside. Now, you can drain the pasta. In that same pot used to cook the pasta, add half of the Bolognese sauce. Add hot drained pasta right on top. Toss the pasta and sauce gently for a couple of minutes, slowly adding the reserved pasta water as needed until the sauce thickens and generously coats the noodles. (I usually only use about 1/2 cup of reserved pasta water, but it varies depending on a lot of factors. Add just enough until you have a thick rich sauce coating the noodles.)