Boil the water. Bring a large pot of lightly salted water to a boil over high heat.
Cook the macaroni. Add the elbow macaroni and cook for 8–10 minutes, or until al dente, following the package directions as needed. Drain the pasta and transfer it to a large saucepan.
Brown the ground beef. Meanwhile, place a large skillet over medium heat. Add the lean ground beef and cook, stirring occasionally and breaking it into small crumbles.
Add the onion. Just before the beef is completely browned, stir in the chopped onion. Continue cooking until the beef is browned and the onion begins to soften.
Combine the beef and pasta. Transfer the cooked ground beef and onion mixture into the saucepan containing the drained macaroni. Stir gently to combine.
Add the tomato ingredients. Pour in the tomato sauce and add the stewed tomatoes, including their juices.
Add the mushrooms and seasonings. Stir in the drained mushrooms, minced garlic, salt, and black pepper. Mix until the pasta and beef are evenly coated with the tomato mixture.
Simmer gently. Place the saucepan over low heat and cook for 10–15 minutes, stirring occasionally, until the mixture is heated through and the flavors are well combined.
Serve warm. Taste and adjust the salt and pepper if needed. Spoon the Slumgullion into bowls or onto plates and serve immediately.