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Heat the oil in a saucepan over a medium heat and fry the onion, pepper and a pinch of salt for 10 mins until softened. Add the cumin, paprika and garlic, and cook for 1 min more. Stir through the beans and season to taste, then tip the mixture into a roughly 20cm baking dish, spreading it to the edges with a spatula.
For the queso, melt the butter in the same pan over a medium heat until foaming. Tip in the flour and turmeric, and cook for 2 mins. Gradually whisk in the milk and cook for 3 mins, stirring continuously. Tip in both the cheeses and whisk until the mixture is thick and the cheese has melted. Pour the queso over the beans, then scatter over the jalapeños and chillies. Serve with tortilla chips for dunking.