Heat vegetable oil in a large pot or Dutch oven over medium heat.

Add chopped onion and cook until softened, about 5 minutes.

Add minced garlic and cook for another minute until fragrant.

Add diced tomatoes (with juice) and vegetable broth to the pot. Bring to a boil.

Gently add cubed tofu to the boiling broth.

Season with fish sauce, sugar, and black pepper. Stir well.

Reduce heat to low and simmer for 10-15 minutes to allow flavors to meld.

Garnish with chopped scallions before serving.