1. Bring a large pan of salted water to boil. Scrub the potatoes, then cut each into 8 wedges. Cook for 6 minutes, then drain and leave to cool slightly in a colander.
2. Preheat the oven to 425 degrees. Divide the oil between two large baking trays. Add half of the potatoes onto each, then sprinkle the salt and paprika over them. Use a spatula to spread the wedges out and cover each in the oil, salt and spice. Bake for 30 minutes, turning once or twice to allow for even cooking, then leave to cool on the trays.
3. Make the cheese sauce: Melt the butter in a large saucepan, then add the flour and stir constantly for 1 minute. Gradually add the milk, whisking thoroughly after each addition to eliminate lumps. Once all of the milk has been added, mix in the sour cream, tomato paste and half of the chopped jalapeños. Once smooth, add half of the cheese and stir until melted before removing from heat.
4. Assemble the pie: Preheat the oven to 375 degrees. Scatter a handful of the remaining grated cheese into the flan dish to help the wedges stay put. Arrange the wedges in a circular pattern in the dish. 5. Pour in the cheese sauce, then top with the remaining cheese and jalapeños.
6. Bake for 15 minutes until warmed through and golden brown on top. Serve immediately.