Season the pork with salt and pepper. Heat 2 tablespoons of oil in a heavy bottomed stock pot over medium-high heat. Add the pork, and cook, stirring occasionally, until the pork is just browning, about 5 minutes.
Add the scallions, ginger, garlic, and onion and saute for 3 more minutes until very fragrant. Add the tomatoes, chicken broth, water, fish sauce, star anise, and cinnamon stick to the pot. Bring to a boil, then reduce the heat and simmer for 20 minutes.
Meanwhile, soak the rice noodles in boiling water for about 10 minutes, until tender. Drain and set aside.
When the soup is done simmering, remove the cinnamon stick and star anise and stir in the juice of 1 lime. To serve, add rice noodles to your bowl, then ladle some soup over the noodles. Top with bean sprouts, cucumber, fresh herbs, fresh lime juice, and a drizzle of siracha. Serve immediately.