1. Warm a large pan over medium-high heat. Add beef and cook, stirring and breaking it up, until beginning to brown, about 5 minutes. Drain off all but 1 Tbsp. of fat, if necessary. Add mushrooms, onion and garlic; sauté for 5 minutes. Stir in tomato puree, broth, Italian seasoning, bay leaf and 1/2 tsp. each salt and pepper. Bring to a boil, then reduce heat to lowest setting and simmer for 45 minutes, stirring often.
2. Bring a large pot of salted water to a boil and cook spaghetti until al dente, about 10 minutes or as package label directs. Remove bay leaf from sauce and season with additional salt and pepper, if desired. Pour sauce over spaghetti, toss and serve.