Combine coconut milk, broth, red Curry paste, lime juice, tamarind paste, lemongrass, fish sauce, sugar, kaffir lime leaves, basil sprig and garlic in a medium saucepan over medium-high; bring to a boil, stirring occasionally. Cover, and remove from heat; let stand for 30 minutes. Pour through a fine wire strainer into a bowl; discard solids.

Melt butter in a saucepan over medium-high. Gradually whisk in the flour constantly until no lumps remain. Slowly whisk in coconut milk mixture; cook, stirring often, until thickened, 5-7 minutes. Remove from heat.

Preheat oven to 400°. Scatter rice evenly on bottom of a buttered 9"x13" baking dish. Top with sliced onion, bell pepper and chicken.

Pour coconut milk mixture evenly over chicken-rice mixture. Cover with aluminum foil and bake until chicken is cooked through, about 1 hour. Garnish with Thai basil leaves.

Substitute for tamarind paste: 3T Worcestershire sauce, 2T water, 3T lemon juice, 2T brown sugar, 1/2C tomato paste.

Substitute for kaffir leaves: lime zest though the fragrance isn't as intense.