Put a large pan of water on to boil. Then, peel the potatoes and cut into thin slices. Put into the pan of boiling water and simmer for about 6 minutes, or until the potato slices are tender.
While the potatoes are boiling, whisk the eggs. Add the yogurt/cream slowly, whisking all the time. Season with salt and pepper and set this to one side.
Place a frying pan on the stove. Put a little oil in the frying pan. Slice the onions roughly and crush the garlic. Put into the pan and fry gently until the onions are just softening. Turn off the heat.
Grease a large pyrex dish with olive oil. Drain the potatoes and lay about half of the slices in the pyrex dish. Top with half of the onion and garlic and a layer of spinach. Pour over some of the egg mixture, keeping back more than half. Layer the rest of the potatoes, onions and spinach. You could add whatever else you have to use up – ham/bacon pieces work really well.
Pour over the rest of the egg mixture and put into the oven. Bake for about 20 minutes or until the eggs are completely set and the top is just beginning to brown. If desired, slice a tomato and add the slices to the top after about 15 minutes into the cooking time.
Once the tortilla is ready, cut it into squares and serve warm with simple salad leaves and some crusty bread and butter. Any leftovers are great cold and popped into lunchboxes the next day.
Notes
Store tortilla Spanish in the fridge for a maximum of 5 days.
Freeze it to extend its storage life for up to 1 month.
Warm it up in the microwave, ensuring you check it every 30 seconds or as needed. Or, you can fry it in a pan for a few minutes.