Preheat the oven to 350 degrees F. Tubers have a high heat capacity, so this dish will take very long to bake.

Bring a large pot of water to a boil over medium heat. Wash and slice the potatoes (you can keep the skins on). Add the potato slices to the boiling water and cook until they are just tender. Drain and reserve.

Meanwhile, in a medium nonstick saute pan, heat the olive oil. Thinly slice (not dice) the onions and add to the pan. Season with salt and black pepper, to taste, and cook the onions until they are soft and starting to caramelize, about 5 minutes. Slice the chorizo (Spanish sausage) and add with the garlic to the pan; cook until the sausage starts to brown. Remove the pan from heat.

In a large bowl, beat the eggs until they are light yellow in color. Stir in the milk, green onions, and shredded Cheddar and combine until incorporated. Adjust the seasonings with salt and black pepper. Add the chorizo mixture and potatoes to the bowl and stir gently. Return the entire mixture back to the saute pan. Put the pan in the oven and bake until the eggs set, about 15 minutes. Remove from the oven and let the tortilla rest in the pan 10 minutes before serving.