How to Make It
In each of 2 large skillets, melt 1 tablespoon of the butter and 1/2 tablespoon of the olive oil over medium heat. Add the onions and garlic to one pan and the ground meat to the other. Season each pan with 3/4 teaspoon salt and pepper. Cook until the onions and garlic have softened and are lightly browned, about 5 minutes. Add the tomato paste to the onions and cook for another 3 minutes. Cook the ground meat until lightly browned, about 6 minutes.
Transfer the onion mixture and the meat to a slow cooker. Add the chopped tomatoes and their juices, thyme, oregano, basil, and the bay leaves. Cover and cook for 4 hours on high or for 8 hours on low. Discard the thyme sprigs and bay leaves.
Bring a large pot of salted water to a boil; cook the spaghetti until al dente. Drain and toss with the sauce. Transfer to plates, and top with freshly grated Parmesan cheese, a drizzling of olive oil, and chopped basil.