In a large skillet, brown the ground beef over medium-high heat.
Make sure you break up the beef with a wooden spoon.
Once browned, drain excess fat.
Add the chopped onion and minced garlic to the skillet.
Continue cooking until the onion becomes tender and translucent.
Pour in the ketchup, water, French onion soup, and yellow mustard.
Stir the mixture well.
Bring it to a simmer.
Sprinkle the brown sugar and flour over the beef mixture.
Stir until the mixture thickens and is well-combined.
Let the mixture simmer gently for about 10 minutes, allowing the flavors to meld together.
Serve the sloppy joe mixture over toasted buns.