In a heavy saucepan, saute the onion in the olive oil until soft and translucent - add the garlic, paprika, and Chipotle salt and fry for another minute. Add a splash of vinegar to deglaze the pan. Add the sugar followed by the tomato. Season with salt and pepper, place the lid on and simmer on a low heat for 1 hour. If you do not want to use it immediately - pour while hot into a clean, sterilized jar - allow to cool and keep in the fridge for one month.
In a heavy-based pan, heat the olive oil and butter. Add the onions with a generous grinding of salt and gently fry the onions until soft and translucent. Turn the heat down and keep stirring occasionally so that they do not catch and burn. Add the water and continue cooking until deep golden brown and caramelized. PS - adding salt to the onions early releases liquid and helps soften them without browning too quickly.
Preheat your oven to 180°C.
The jury is out on whether butter or mayo gives you the best toastie! We're in the mayo camp but either way, spread each slice of bread with your preference of butter or mayo, ensuring that you evenly cover the entire slice for even browning.
Top with a small grinding of salt. Place 2 slices buttered side DOWN. Spread each slice with a tbsp tomato relish. Top with caramelized onions, followed by the grated cheese. Season with pepper.
Finally top each one with the remaining slices of bread, buttered side UP. Using a spatula, carefully place in a heavy based non-stick pan or skillet and gently fry over medium heat until beautifully golden brown on both sides and the cheese has completely melted. If the cheese has not melted or if you're feeding a crowd - place onto a baking sheet in a pre-heated oven for 5-10 minutes until hot, crispy and melted! Slice in half and serve immediately!