1. Combine first 6 ingredients.
2. Cut baguette crosswise into 4 pieces. Cut each piece in half horizontally using a serrated knife. Spread 1 teaspoon butter on each bottom half of baguette. Divide roast beef evenly over bottom halves of baguette. Arrange about 1 cup slaw mixture and 2 tomato slices over each sandwich; top with top halves of baguette.