Roast 30 minutes. Reduce oven to 300 degrees and continue roasting beef 70 to 75 minutes, or until a meat thermometer registers 135 degrees for medium-rare, or until desired doneness beyond that.

Spread other 6 slices with about 1 teaspoon each of the cream cheese spread. Top with roast beef, dividing evenly. Gently press avocado-topped bread slices, avocado sides down, onto each sandwich.