Grilled Steak Sandwiches with Marinated Watercress, Onion, and Tomato Salad Recipe | Epicurious

Ingredients

• 1/2 cup olive oil
• 6 tablespoons white balsamic vinegar
• 1 3/4 teaspoons dry mustard, divided
• 1 tablespoon dark brown sugar
• 1 2-pound well-trimmed tri-tip beef roast
• 4 large plum tomatoes, very thinly sliced
• 1 bunch watercress, stems trimmed
• 1/2 white onion, very thinly sliced
• 6 5-inch pieces French baguette, cut horizontally almost in half, opened flat