Lightly season shrimp with salt, set aside. Heat oil in a 5 to 6-quart Dutch oven over medium-high heat. Add bell pepper and cook, stirring util almosst tender, about 4 minutes. Add white part of scallions, 1/4 cup cilantro, garlic and pepper flakes. Continue to cook, stirring, until fragrant, another minute. Add tomatoes and coconut milk and bring to a simmer. Reduce heat to medium and simmer to blend flavors and thicken sauce slightly, about 5 minutes. Add shrimp, partially cover, and continue to cook, stirring frequently, until just cooked through, about 5 more minutes. Stir in lime juice and adjust seasoning adding salt, if necessary.

Serve over sticky rice, mounding rice on plate, ladle shrimp on top or alongside, garnish with scallion greens and remaining 1/4 cup of cilantro.