1. Put the shrimp into a large bowl, season with salt and pepper, then mix in the lemongrass. Heat 1/4 cup of the olive oil in a large, deep skillet over moderate heat. Add the shrimp, stirring and tossing them with a spatula. Saute until they turn pink, about 2mins. Remove the shrimp from the pan and set aside while you make the sauce.
2. Into the same skillet with the oil and shrimp juices, put the remaining 1/4 cup olive oil and the onions, garlic, celery, and bell peppers and cook, stirring constantly with a wooden spoon for about 2mins.
3. Add the tomatoes. Reduce the heat to medium-low, and, when the sauce comes to a simmer, add the bay leaf, allspice, and red pepper flakes. Simmer for 10mins.
4. Add the shrimp back to the skillet along with the basil and mint. Cook for a minute or two. Season with salt and pepper. If the sauce tastes too tart, add a little sugar to balance the flavor. Remove the bay leaf. Serve over steamed white rice.