Peel, halve and thickly slice potatoes. Place in a microwave safe bowl, cover with water and microwave on high for 8 minutes or until just tender. Drain and reserve.
In the meantime, heat oil in a large, oven proof, non-stick frypan (my frypan measures 11inches/27.5cm across the top) over low/medium heat.
Add the onion to the pan and fry for 5 minutes until softened but not coloured.
Add sliced chorizo sausage. Fry for 5-8 minutes until the fat in the chorizo has rendered. You may want to carefully drain the fat if there is a lot.
Add the potato and toss to combine. Fry the potato in the onion/chorizo for 3-5 minutes. The potato will absorb the flavours. Season with salt and pepper.
In a large bowl, beat eggs. Then add grated tasty cheese, grated parmesan, chopped parsley, milk and salt and pepper to season.
Pour egg mixture evenly over potatoes. Cook over gentle heat for 5 minutes until starting to bubble around the edges.
Then put into the oven to complete cooking approx. 15-25 minutes.
Frittata will be ready when you press into the middle and it’s firm; push the blade of a knife into the middle and no gooey egg sticks to the blade; the frittata will be browned and smell delicious.
Potato Frittata can be made ahead at different stages.
Make ahead to Step 6. Have potatoes and chorizo mixture prepared. Refrigerate, well covered overnight or even a day or two in advance. When ready to complete the frittata proceed with Step 7 and finish cooking.
Make ahead completely. Cool to lukewarm and refrigerate. When cold cover well with plastic wrap or other covering. The frittata will keep for a couple of days. Reheat in a low oven or eat cold.
No ovenproof, non-stick frypan? Use a well-seasoned pan of similar dimensions. Or transfer the cooked potato and chorizo mixture from Step 6 to an buttered, ovenproof baking dish. A good size would be 23 x 33cm (9 x 13 inch).What if you don't have access to an oven?Frittata can be completely cooked on the stove top. After adding the egg, reduce the heat to low, cover with a lid and cooked gently until the egg is set. Be sure to turn the heat to low and keep a close eye on it so that the bottom doesn't burn.