Toss beef with Hellmann's® or Best Foods® Real Mayonnaise in medium bowl and let stand 30 minutes to marinate.
Remove beef from marinade, then lightly roll in flour.
Heat 1 Tbsp. oil in large skillet and brown beef over medium-high heat, stirring occasionally, about 5 minutes. Remove beef from skillet and add to slow cooker.
Heat remaining 1 Tbsp. oil in same skillet over medium heat and cook onion, carrots, celery, leek, mushrooms and garlic, stirring occasionally, until onion is golden, about 4 minutes. Stir in tomato paste and wine. Bring to a boil, stirring occasionally.
Add vegetable mixture to slow cooker, then stir in Knorr® Homestyle Stock - Beef blended with boiling water. Cook covered at HIGH 4 to 5 hours or at LOW 8 to 10 hours or until beef is tender. Serve, if desired, with mashed potatoes.