In a medium skillet over low heat, add the coriander and turmeric and toast until aromatic, about 2 minutes.
Put the rendang sauce in a medium bowl. Add the toasted coriander and turmeric and mix in the coconut milk.
Place the skirt steak in a large baking dish. Pour the rending marinade over the steak and turn to coat evenly. Cover with a plastic wrap and refrigerate for 3 hours or as long as overnight.
When ready to cook, heat a grill or grill pan over medium-high heat. Lightly oil the grill grates.
Grill the steak, turning occasionally, for 6 to 8 minutes for medium rare, or 12 minutes for medium. Transfer to a cutting board and let rest for 10 minutes. Slice the steak across the grain and divide the slices evenly among the lettuce leaves. Serve with a side of sriracha sauce for dipping, if desired. Serve immediately.