Remove potato skin with a speed peeler (Y-slicer). Then, peel the potatoes into thin slices.
Place sliced potatoes into a large bowl, submerge with cold water and mix well. The colour of the water should become cloudy as the starch is washed out of the potato. Strain the water, then repeat. Leave potato submerged in cold water for 15 minutes. After 15 mins, strain the potatoes and return to the bowl.
Peel, halve and finely slice the white and red onion. Also dice the green or red chilli and peel and finely slice the garlic cloves.
Bring a full kettle of water to the boil. Sprinkle 2 tsp fine sea salt over the potato slices, pour over the boiling water and mix thoroughly to ensure salt is evenly dissolved and distributed. Leave the potatoes to blanch for 5 minutes.
Bring a large non-stick frying pan to a medium-high heat, and add 3 x tsbp of olive oil. Add the onions and sweat for 3-4 minutes, or until softened. Add the green chilli and then the garlic, reduce heat slightly, then fry for a further 2-3 minutes.
After 5 minutes, strain the potatoes in a colander and try and squeeze out as much water as possible. Transfer to a clean kitchen towel, and try and wring out as much of the water as you can.
Turn up the heat slightly, then add the potato slices to the pan. Mix all the ingredients thoroughly, then season well with freshly ground sea salt and black pepper. Add the fresh thyme (leaves and thin stalks are fine), then keep stirring through for a few more minutes.
Break eggs into a small mixing bowl, drop in a few small dollops of Dijon mustard, then whisk thoroughly.
Flatten the potato mixture, then pour over the beaten egg and distribute it evenly on top. Season once more with salt, pepper, and thyme leaves. Leave the tortilla to cook on a medium-high heat until it has set. You can give the pan a little shake to check if it has come together. The top will still be soft/ a little liquid, but that is fine.
Preheat grill to 200C, then transfer the frying pan and grill the tortilla for a further 3-4 minutes, or until the top is firm and browned.
Using a pan turner of fish slice, divide into 4 then flip over only a plate and serve.