Combine mayonnaise and next 6 ingredients in a medium bowl. Add trout; toss gently to combine. Spread each half of the rolls with 1 tablespoon butter. Arrange the watercress and tomatoes evenly on each roll half. Top with trout salad, and serve immediately.
Wine note: "Wow! White Burgundy's lemon flavors strike amazingly together with the red onion, capers, and smoked trout. I select Oliver Leflaive Les Setilles White Burgundy 2003 [$16]. This is 100 percent Chardonnay. Surprise--there is no oak. The wine is fermented in stainless steel barrels. Look for a taste of Granny Smith apples." --Rosalind Johnson, Divas Uncorked, Massachusetts