Place flour in baking pan. Season with salt and pepper. Melt 4 tablespoons butter in heavy large Dutch oven over medium-high heat. Working in batches, coat meat with flour; add to pot and brown on all sides. Using slotted spoon, transfer to plate.
Melt 2 tablespoons butter in same pot over medium-high heat. Add onions; saute until tender, about 6 minutes. Mix in tomato paste, then wine. Bring to boil , scraping up any browned bits. Add broth and sugar, then beef and any accumulated juices . Bring to boil. Reduce heat, cover partially and simmer 1 1/2 hours.
Add potatoes and carrots; simmer uncovered until meat and vegetables are almost tender, about 25 minutes. Add squash; simmer until almost tender, about 10 minutes. Add mushrooms and 3 tablespoons marjoram; simmer until mushrooms are tender, about 5 minutes. Season with salt and pepper. (Can be made 1 day ahead. Cool slightly. Cover, chill. Before serving, cook over medium heat, stirring occasionally.)