Brown the ground beef, onion, and garlic in a skillet over medium-high heat until beef is cooked through and onion is soft. Drain fat.
Transfer beef mixture to a 4-quart or larger slow cooker. Add diced tomatoes with juice, pasta sauce, chicken broth, basil, and parsley. Stir to combine.
Cover and cook on high for 3 hours, until the sauce deepens and thickens.
Stir in dry mostaccioli and push pasta down into the sauce. Cover and cook on high for 20 minutes.
Stir thoroughly from the bottom up, then cook 15 to 20 more minutes, until pasta is tender.
Sprinkle mozzarella over the top, replace lid, and let sit on high for 5 minutes until melted. Serve immediately.
Notes
Add the pasta dry near the end so it cooks in the sauce without getting mushy. Stir halfway through the pasta cook time so the bottom noodles do not stick. Use chicken broth instead of water for better flavor. Serve right away because the pasta continues to absorb sauce as it sits.