Combine 1/4 cup flour, oregano, salt, and pepper in a bowl. Add beef and toss to coat.
Heat olive oil in a cast iron skillet over medium-high heat heat. Brown beef in the hot oil on all sides, 5 to 7 minutes. Transfer meat to the bottom of a slow cooker. Do not clean the skillet.
Pour 1/2 cup red wine into the skillet and add pearl onions and mushrooms. Saute over medium-high heat until softened, 5 to 7 minutes. Be sure to scrape the meat bits into the mixture with a wooden spoon. Reduce heat and simmer for 20 minutes.
Stir potatoes, carrots, and celery into the slow cooker. Add beef broth and remaining wine. Season with salt and pepper.
Cover and cook until beef is no longer pink in the center and vegetables are tender, on Low, 6 to 8 hours, or High, 4 to 6 hours. Near the end of cooking time, whisk boiling water, remaining flour, and bouillon together; stir into the pot and allow sauce to thicken.