Heat the oven to 200C/fan 180C/gas 6. Soften the onions in the oil over a low heat for about 25-30 minutes until really soft and squidgy. Stir in the sugar and keep cooking for a few minutes more until golden brown. Roughly chop 2 tbsp of the onions and set aside in a mixing bowl.
Mix the grated cheese and mustard together. Take a roasting tin or large baking dish and layer up the tortilla chips with the caramelised onions, chorizo slices, jalapeños, chilli (if using) and blobs of the mustardy cheese.
Bake for 10-15 minutes until the cheese has melted and tortilla chips toast at the edges.
Meanwhile mix the diced tomatoes, reserved chopped onions, ketchup and lime juice. Dollop the salsa over the baked nachos, then drizzle with the soured cream – loosened with a teaspoon or two of water. Squirt over a bit more american mustard if you like, and eat immediately.