In a large skillet over medium heat, heat the olive oil. Once warmed, cook the onion until softened and beginning to turn translucent and golden, about 7 minutes. Add the garlic and cook a minute more. Transfer to the slow cooker.
In the same skillet, brown the beef, working in batches as necessary. Drain all but about 2 tbs of the fat, and transfer to the slow cooker.
Deglaze the pan with the red wine, scraping to remove any browned pits. Bring to a simmer, then transfer to the slow cooker.
Add the remaining ingredients except the spaghetti to the slow cooker, then cover and cook on low for 6 hours.
When ready to serve, cook the spaghetti per package directions. Before draining, reserve 1/2 cup of the pasta water.
Just before serving, toss the pasta with about 2 cups of the bolognese, along with the pasta water. The starch in the pasta water helps this layer of sauce stick to the pasta. When serving, top the tossed spaghetti with more sauce, as desired, as well as freshly grated Parmesan.