In a large bowl, combine 1/3 cup flour, garlic powder, nutmeg, 2 teaspoons salt and 1/4 teaspoon black pepper. Pat the beef dry with paper towels and add to the flour mixture in the bowl; toss to coat.
In a Dutch oven, heat the oil over medium-high heat. Working in batches (don't crowd the pan), add the beef and cook until browned on all sides. Transfer to a plate.
Reduce the heat to medium and add the onion. Cook, stirring often, until softened, about 4 minutes. Add the brandy and wine and cook, scraping the pan with a spoon, about 1 minute. Return the meat to the pot and add the beef broth. Bring to a boil, reduce heat to low, cover and simmer until the beef is tender, about 1 1/2 hours.
Transfer one cup of broth to a small bowl; add the remaining 3 tbsp. flour and whisk until smooth. Add the flour mixture to the pot. Season to taste.
In a large skillet, melt the butter over medium heat. Add the mushrooms and cook, stirring often, until golden brown, 5-7 minutes. Add the mushrooms to the meat mixture. Simmer 30 minutes longer.
Stir in the sour cream and season to taste with salt and pepper.
Serve stroganoff over buttered noodles. Garnish with additional sour cream, plus chopped chives.
Notes
Store leftover stroganoff in an airtight container in the refrigerator for up to 3 days, or in the freezer for up to three months. Let it thaw overnight in the fridge before reheating. Reheat over moderate heat on the stove, or in the microwave.