Cream the butter and cheese with a fork in a medium bowl until evenly mixed together.
Lay the roast beef slices on a flat surface to make 16 rectangles, each about 5 by 8 inches. Depending on the size of the slices, it may be necessary to combine them, overlapping slightly along the sides. Season the meat with salt and pepper. Spread 2 teaspoons of the cheese mixture along the short end of the beef rectangle. Strip the leaves from a watercress sprig and scatter the leaves over the cheese, and then top with a few pieces of both red and yellow peppers. Roll the beef, enclosing the butter and vegetables, and continue rolling into a tight cylinder. Wrap snugly in plastic wrap. Repeat with the remaining ingredients. Refrigerate until chilled, at least 30 minutes.
To serve, trim the ends and halve the rolls, crosswise.
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