How to Make It
Light a grill or preheat a grill pan. Brush the medallions with oil and season with salt and pepper. Grill over high heat, turning once, until lightly charred on both sides and medium-rare within, about 6 minutes. Transfer to a work surface and let rest for 5 minutes.
Meanwhile, in a small bowl, whisk the mayonnaise with the sesame oil and 1/2 teaspoon of Sriracha and season with salt and pepper. Brush the cut sides of the baguette with oil and grill over moderate heat until toasted, about 2 minutes.
Slice the medallions 1/4 inch thick. Spread the sesame mayonnaise on the cut sides of the baguette and top with the meat, pickled watermelon rind and cilantro. Close the sandwiches and serve with additional Sriracha.