Simmer brisket in water to cover till almost tender (about 1 1/2 hours). By this time the water will have boiled down to about half the original amount. Peel carrots and slice in half-inch rounds.
Add carrots, salt and sugar to meat. Bring to a boil, reduce heat and simmer till carrots are very tender (about 45 minutes).
Brown flour in a very small skillet, stirring constantly to prevent burning. When flour is very light brown, remove from burner. Add schmaltz and stir till smooth.
Gradually stir in carrot stock to make a smooth paste. Add this paste to the carrots, stirring it carefully. Continue to simmer till stock has thickened 10 to 15 minutes.