In the stovetop-safe insert of a slow cooker over medium-high heat, cook the bacon until crisp, 7 to 10 minutes. Spoon off all but 1 Tbs. of the fat. Add the onion and 1/2 tsp. of the salt and cook until tender, 5 to 7 minutes. Add the garlic and cook for 1 minute. Add the ground beef and the remaining 1 tsp. salt and cook, stirring occasionally, until the beef is no longer pink, about 5 minutes. Stir in the tomato paste, milk and the 1/3 cup cheese.

Place the insert on the slow-cooker base. Cover and cook on low according to the manufacturers instructions until the sauce thickens, about 4 hours. Skim the fat off the sauce. Adjust the seasonings with salt and pepper.

Add the cooked spaghetti to the sauce along with enough pasta-cooking water to loosen the sauce and toss to combine. Transfer to individual bowls and sprinkle with cheese. Serve immediately. Serves 6 to 8.