Combine the flour, 1 teaspoon kosher salt, and black pepper on a plate or in a shallow bowl. Add the beef cubes and toss until lightly coated.
Heat 1 tablespoon butter in a large skillet over medium-high heat. Add the chopped onion and sauté for approximately 5 minutes, until it begins to turn translucent.
Add the garlic, sliced mushrooms, and remaining teaspoon of kosher salt. Cook for another 3 minutes, stirring occasionally.
Transfer the mushroom and onion mixture to the slow cooker.
Add the vegetable oil to the same skillet. Add the flour-coated beef cubes and cook for approximately 5 minutes, turning occasionally, until the outside develops a browned crust. Work in batches if necessary to avoid overcrowding.
Transfer the browned beef to the slow cooker with the mushrooms and onions.
Pour the red wine into the hot skillet. Use a wooden spoon to scrape up the flavorful browned bits from the bottom of the pan. Simmer for approximately 5 minutes, allowing the wine to reduce slightly.
Pour the reduced wine into the slow cooker and add the beef broth. Stir gently to combine.
Cover and cook on high for approximately 2 hours or low for approximately 4 hours, continuing to cook as needed until the beef is fork-tender. I prefer cooking this on low when I have the time.