In a large skillet, heat the peanut oil. Add the onion and cook over moderately high heat, stirring occasionally, until it begins to brown, about 5 minutes. Add the tomato paste mixture and cook, stirring constantly, until fragrant, about 2 minutes. Add the shrimp and cook until they are just opaque, about 3 minutes. Add the fish sauce, rice vinegar, sesame oil and half of the scallions. Remove from the heat.