Preparing the Onion and Spinach

1. While the potatoes are boiling, finely chop one onion.
2. Heat a pan with a little oil over low heat and add the chopped onion.
3. Cook the onion until it softens, stirring constantly to prevent burning.
4. Wash the spinach thoroughly and cut it into thin strips, discarding any damaged parts.
5. Add the spinach to the pan with the cooked onion and cook for 3-4 minutes until the spinach wilts.
6. Remove the pan from the heat and let the mixture cool slightly.

Creating the Dressing

1. In a deep bowl, crack 4 eggs and whisk them together.
2. Add 4 tablespoons of sour cream to the eggs and whisk until well combined.
3. Season the mixture with salt, black pepper, and garlic powder to taste.
4. Add 1 teaspoon of baking powder to the mixture and combine thoroughly.
5. Grate half of the hard cheese and add it to the egg mixture, stirring to incorporate.

Assembling and Baking

1. Grease a baking dish and arrange the boiled potatoes in an even layer.
2. Spread the cooked spinach and onion mixture over the potatoes.
3. Pour the egg and cheese mixture over the spinach layer.
4. Sprinkle the remaining grated cheese over the top.
5. Bake the dish in a preheated oven at 180°C for approximately 20 minutes, or until the cheese is golden and bubbly.

Conclusion
In conclusion, cooking spinach with potatoes is not only a delicious way to enjoy two humble ingredients but also a tribute to the rich culinary traditions of Spain. With simple yet flavorful components, this dish is sure to become a favorite at your dinner table.