In shallow bowl, combine flour, salt thyme, pepper, and rosemary. Coat beef cubes lightly with flour mixture, reserving any remaining flour mixture to thicken stew.
In Dutch oven or stock pot, heat margarine over medium heat. Brown beef cubes on all sides in batches. Add leeks, garlic and mushrooms to browned beef; cook until leeks are soft, about 5 minutes. Gradually stir in broth, scraping up brown bits from bottom of pan. Stir in tomato paste; mix well.
Bring to a boil; reduce heat, cover and simmer for 1 1/2 hours or until meat is tender.
To thicken, whisk 2-3 tablespoons cold water into reserved flour mixture until smooth. Gradually whisk into bubbling stew. Cook and stir until sauce thickens.
Beef and Portobello Pot Pie: Spoon cooled stew into shallow 6 cup casserole. Brush top edge of casserole with slightly beaten egg white. Roll out half a (397g) package, defrosted purchased puff pastry to a size that is 1-inch larger than casserole diameter. Place pastry over stew, sealing pastry to edge of casserole. Cut steam vents into pasty. Bake in 400F oven for 30-35 minutes, or until filling bubbles, and pastry is puffed and golden. (If stew is made ahead and refrigerated, increase baking time by 10-15 minutes).