Gather the ingredients so you have everything close at hand on the counter or table.
Prepare the Ingredients
Using a knife and cutting board, dice the sweet onion, and chop the mushrooms into small pieces. Mince the garlic.
In a large skillet, brown sausage. Transfer sausage to a bowl, and drain any excess grease.
In the same skillet, sauté prepared onion, mushrooms, and minced garlic in olive oil until tender, about 1-2 minutes.
Make the Red Sauce
In a large slow cooker, combine diced tomatoes, tomato paste, dry wine, Worcestershire sauce, red wine vinegar, granulated sugar, bay leaf, dried parsley, basil, oregano, kosher salt, and freshly ground black pepper.
Add sautéed onion, mushrooms, and garlic to the sauce. Stir to combine.
Next, add the browned sausage and stir to combine. Cover and cook on low for 7-8 hours, or on high for 3 1/2 – 4 hours, stirring once or twice.
Once sauce is nearly done, prepare dried spaghetti as directed.
When ready to serve, add drained spaghetti pasta to the sauce and stir with a long-handled spoon until fully incorporated. Alternately, you may serve the spaghetti sauce on individual plates over cooked spaghetti.