Heat a grill to medium high. Bring a medium pot of water to a boil. Add the noodles and cook as the label directs. Drain and rinse under cold water. Set aside in a colander to drain thoroughly.
Whisk the lime juice, 3 tablespoons water, the sugar, sambal oelek and fish sauce in a large bowl. Add the cucumber, onion, watermelon and noodles. Season with salt and pepper and toss; set aside.
Season the steak with salt and pepper. Brush the grill with vegetable oil. Grill the steak until marked, about 3 minutes per side for medium rare. Transfer to a cutting board and let rest 5 minutes, then thinly slice.
Add all but a few tablespoons cilantro to the noodle salad and toss. Top with the steak and the remaining cilantro.
Photograph by Antonis Achilleos
Recipe courtesy of Food Network Magazine