Meanwhile, in a medium non-stick oven-safe pan, heat the olive oil and add the onions. Season with salt and black pepper, to taste, and cook the onions until they are soft and starting to caramelise, about five minutes.
Drop in the sliced chorizo and the garlic and 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 colour. Stir in the milk, spring onions, and 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 pan. Put the pan in the oven and bake until the eggs set, about 15 minutes.
Remove from the oven and allow the tortilla to rest in the pan for ten minutes before inverting. Using a large clean plate as an aid, flip the tortilla over and slide it out onto a large cutting board.
Slice the tortilla into wedges and arrange on serving dishes. Serve it warm or at room temperature.