Bring water to a boil for pasta. Chop onion finely.
Melt butter in large sauté pan on medium-high, then add onions and garlic; cook and stir 1 minute.
Add beef, mushrooms, salt, and pepper to pan; cook 5–7 minutes, stirring to crumble meat, until no pink remains and meat is 160°F.
Meanwhile, cook pasta following package instructions.
Add flour to pan; cook and stir 2 minutes.
Stir broth slowly into beef mixture, then stir in sour cream.
Reduce heat to medium; cook and stir 7–8 minutes to blend flavors.
Drain pasta; serve beef mixture over pasta.
Drain 1 can beans. Combine all ingredients (including liquid from remaining 1 can beans) in microwave-safe bowl.
Cover and microwave on HIGH 6 minutes, stirring halfway through cook time, or until thoroughly heated.
Stir and serve.
