Dredge beef in 2 tablespoons all-purpose flour.
Brown beef in hot oil in a large Dutch oven, and remove from pan. Wipe pan clean.
Melt butter in Dutch oven; add onion, and saute until tender. Sprinkle with remaining 1 tablespoon flour, and stir gently. Add beef, broth, and red wine; bring to a boil. Reduce heat, and simmer 35 to 40 minutes.
Ladle into ovenproof soup bowls; top each with a baguette slice, and sprinkle evenly with cheese. Place bowls on baking sheets.
Broil 5 inches from heat (with electric oven door partially open) until cheese melts. Serve immediately.