In a lg. bowl,sprinkle the shrimp with 1tsp. salt;toss to coat and set aside.
Heat the oil in a 5to6qt. dutch oven over med-high heat. Add the bell pepper and cook,stirring,until almost tender,about 4 mins. Add the scallion whites,1/4c of the cilantro,the garlic and the pepper flakes. Continue to cook,stirring,until fragrant,30 to 60 seconds.
Add the tomatoes and coconut milk and bring to a simmer. Reduce the heat to medium and simmer to blend the flavors and thicken the sauce slightly,about 5 mins.
Add the shrimp and continue to cook,partially covered and stirring frquently,until the shrimp are just cook through,about 5 mins. more.Add the lime juice and season to taste with salt. Serve,sprinkled with the scallion greens and remaining 1/4c cilantro.