In a large bowl, mix the garlic powder with 1 1/2 tsp. salt and 1/4 tsp. pepper. Pat the beef dry with paper towels and toss to coat in the garlic powder mixture. Add 1/3 cup flour and toss again.
In a Dutch oven, heat the oil over medium- high. Working in batches, add the beef in a single layer, spacing 1/2 inch apart, and cook, turning, until browned on all sides, about 5 minutes per batch. Transfer to a plate. Reduce the heat to medium and add the onion. Cook, stirring often, until the onion starts to soften, about 2 minutes. Add the cognac and cook, scraping up any browned bits on the bottom of the pot, about 1 minute. Return the meat to the pot and add the stock. Bring to a simmer and reduce the heat to low. Cover and simmer, stirring occasionally, until the beef is almost tender, about 1 1/2 hours.
Transfer 1 cup stock from the pot to a small bowl. Add the remaining 3 tbsp. flour and whisk until smooth. Add the mixture back to the pot.
In a large skillet, melt the butter over medium heat. Add the mushrooms and cook, stirring often, until golden brown, 5 to 7 minutes. Add the mushrooms to the beef mixture. Simmer until the meat is tender, about 30 minutes longer.
Stir in the sour cream; season. Place the egg noodles in a serving dish; top with the beef mixture. Garnish with the chives.