Add oil to a heated large pot over medium heat.
Sauté the lemongrass and onions for 3 minutes on low heat, stirring occasionally.
Stir in mushrooms, ginger slices, chili pepper, and Panang curry paste.
Saute and cook for 5 minutes on low heat.
Add the vegetable stock, coconut milk, tomatoes, garlic, fish sauce, and brown sugar.
Stir to incorporate with the soup fully.
Bring to a boil, then reduce to a simmer and cover for 20 minutes.
Season with salt and pepper.
Turn off the heat and add a splash of lemon juice and zest.
Serve and top with fried tofu.