• Step 1: In a large Dutch oven or heavy-bottomed pot, heat the olive oil over medium-high heat. Working in batches if necessary, brown the beef cubes on all sides, about 2-3 minutes per batch.
Transfer the browned beef to a plate and set aside.
• Step 2: Reduce the heat to medium and add the diced onion to the pot. Cook, stirring occasionally, until the onion is softened and translucent, about 5 minutes. Add the garlic and cook for an additional minute, until fragrant.
• Step 3: Deglaze the pot with the red wine, scraping up any browned bits from the bottom of the pot. Bring the wine to a simmer and cook for 2-3 minutes to allow the alcohol to cook off.
• Step 4: Add the beef broth, bay leaves, thyme, tomato paste, and Dijon mustard to the pot. Stir to combine, then return the browned beef and any accumulated juices to the pot. Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for 1.5 to 2 hours, or until the beef is very tender.
• Step 5: In a separate skillet, melt the butter over medium heat. Add the pearl onions and cook, stirring occasionally, until lightly browned, about 5-7 minutes. Sprinkle the flour over the onions and stir to coat. Cook for an additional 2-3 minutes, then add the onions to the beef stew.
• Step 6: Add the sliced mushrooms to the pot and continue simmering, uncovered, for an additional 30 minutes, or until the sauce has thickened to your desired consistency. Season with salt and pepper to taste.