Heat the oil in a large pan over medium-high heat, then add the red onion and cook for 5 minutes, until browned.
Add the tomato, serrano, and garlic. Stir well, cover, and cook for 5 minutes, until the tomatoes have softened.
Add the garam masala, turmeric, and cumin. Stir, reduce the heat to medium, and cook for another 10 minutes, or until the sauce clings to the potatoes and they soften slightly.
Stir in the lemon juice and cilantro, then add the fries and salt. Toss well to coat. Serve immediately.
These are absolutely delicious. I added a bit of cayenne pepper in mine to add more spice.