Heat oil in a large frying pan over medium-high heat.
Add onions. Saute, stirring occasionally, for about 2-3 minutes or until soft and translucent.
Add garlic and ginger. Saute, stirring, for about 1 minute or until fragrant.
Add cumin, red chili powder and chat masala. Saute, stirring, for about 30 seconds or until aromatic.
Add tomato paste and ketchup (tomato sauce). Stir to combine. Season with salt.
Cook, stirring occasionally, for about 1-2 minutes.
Add cooked fries just before serving. Toss.
Garnish with chopped coriander (cilantro) leaves.
Drizzle with fresh lemon juice.