In a large pot, heat the vegetable oil over medium heat.
Add the beef or pork pieces and cook until browned on all sides. Remove from the pot and set aside.
In the same pot, add the chopped onion, minced garlic, and minced ginger. Sauté until the onion is translucent and fragrant.
Stir in the red curry paste and cook for 1-2 minutes.
Return the browned meat to the pot. Add the beef or chicken broth and water. Stir well to combine and bring to a boil.
Reduce the heat and let the soup simmer for about 30 minutes, or until the meat is tender.
Add the bamboo shoots, mushrooms, and chopped tomato to the pot. Continue to simmer for another 10-15 minutes.
Season the soup with fish sauce, lime juice, and sugar (if using). Adjust the seasoning to taste. Add sliced Thai bird chilies for heat.
Garnish with fresh cilantro and green onions.
Enjoy your meal!