Trim excess fat from roast. Then, cut 8 evenly spaced crosswise slits or pockets in roast, making each 3 inches deep. Place roast in a plastic bag set in a deep bowl. Pour wine over roast. Seal bag. Marinate roast in the refrigerator for 6 to 24 hours, turning bag occasionally.
For filling, in medium saucepan combine mushrooms, water, dillweed or fennel seed, and salt. Cook, uncovered, over medium heat for 15 minutes or until liquid is evaporated. Cool. Stir in onion and mayonnaise or salad dressing.
Remove roast from bag, discarding marinade. Spoon filling into pockets in roast. Place roast in a shallow roasting pan. Roast in a 325 degree F oven for 1-1/4 to 1-1/2 hours or until meat thermometer inserted into thickest part of meat registers 140 degrees F. Remove from oven. Keep warm.
For sauce, in a small saucepan stir together yogurt, flour, and pepper. Stir in beef broth. Cook and stir over medium heat until thickened and bubbly. Then, cook and stir for 1 minute more. To serve, slice meat between filled pockets. Serve with sauce. Makes 8 servings.