Preheat the slow cooker if necessary; see the manufacturer's instructions. Lightly oil a 1.2 litre (2 pint) soufflé dish and base-line with a circle of nonstick baking paper, checking first that the dish will fit in the slow cooker pot.
Heat the oil in a frying pan, add the onion and chorizo and fry for 4–5 minutes until the onion is softened. Beat the eggs, milk and a little salt and pepper in a bowl, then add the onion and chorizo, the red pepper and sliced potatoes and mix together.
Tip the mixture into the oiled dish, cover the top with foil and stand the dish in the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the dish. Cover with the lid and cook on high for 2–2½ hours or until the egg mixture has set in the centre.
Lift the dish out of the slow cooker pot using a tea towel and remove the foil. Loosen the edge of the tortilla with a knife, turn it out on to a plate and peel off the lining paper. Cut into slices and serve hot or cold with salad.