Set a large 6-quart slow cooker to low heat. Cut the beef roast into small bite-size chunks, or cut against the grain into thin strips.
Place the butter in a large skillet and set on the stovetop over medium heat. Once melted, add the onions and garlic. Sauté for 2-3 minutes, then add the beef pieces. Brown the beef on all sides, about 5 minutes. Then sprinkle the flour over the beef and stir to coat. Cook another 3 minutes. Pour the mixture in the bottom of the slow cooker. *If using an Instant Pot or Multi-Cooker, do this in the crock on the Sauté setting, then lower the heat to Slow Cook.
Add the chopped mushrooms to the crock. In a bowl, whisk the sour cream, cognac, beef base, Dijon mustard, and thyme together. Pour the mixture over the mushrooms, then add in 3 cups water. *If using less than 1 cup cognac, add extra water to equal 1 cup.
Cover the crock and slow cook for 5-6 hours on high, or 7-9 hours on low, until the beef is fork-tender. Stir in the egg noodles. Cover again and slow cook another 20-30 minutes, until the pasta is cooked to al dente. Taste, then salt and pepper as needed.
If you like stroganoff with a strong boozy essence, use 1 cup cognac. If not, go with 1/2 cup cognac + 1/2 cup water.