Heat oil in a large saucepan over medium heat. Add onion and cook, stirring frequently, until tender, about 6 minutes. Add beef, in batches if necessary, and cook, breaking up with a wooden spoon, until browned. Add remaining ingredients, except spaghetti and cheese. Stir to mix well. Bring to a boil. Reduce heat to low; cover and simmer 1 1/2 hours, stirring occasionally. Best if you now refrigerate overnight.
Remove the bay leaf. Reheat gently over medium heat.
Serve over hot, drained spaghetti. Top with shredded cheddar cheese.
