In a Dutch oven or heavy stock pot over medium heat, saute the butter and onion, just until the onion is softened. Add the curry paste and stir to combine. Then add the garlic, ginger, red bell pepper, and mushrooms. Cook until softened, about 3 to 5 minutes. Add chicken broth, coconut milk, fish sauce, curry powder, brown sugar, red pepper flakes, and chicken. Stir to combine and cook until thoroughly warmed. Before serving, stir in lime juice, green onions, and cilantro. Season with kosher salt and freshly cracked black pepper, to taste. It may take a couple teaspoons of salt to reach good flavor, depending on the saltiness of the rest of the ingredients – just start out by adding a 1/2 teaspoon at a time, tasting after each addition.