In a medium sized skillet, sauté the onion gently in the oil or lard until soft and translucent.
Add the crumbled sausage meat and sauté it, too, until it has lost any trace of pink and is just beginning to brown around the edges, breaking it up further as it cooks.
Add the mushrooms to the skillet, along with a bit more oil or lard if needed. Raise a flame a bit and sauté the mushrooms until they have reduced in volume and just begun to brown around the edges. Now add the potatoes and toss them with the other ingredients, seasoning generously with salt and pepper.
After a couple of minutes, lower the heat and cover the skillet. Let everything simmer gently until the potatoes are tender, uncovering the skillet and stirring its contents every once in a while. Taste and adjust for seasoning before turning into a mixing bowl and let cool.
In another mixing bowl, beat the eggs together with the grated cheese and a pinch of salt and pepper. When the sausage, potatoes and mushrooms have cooled, add them to the bowl and fold them into the beaten eggs.
Now proceed to cook the egg mixture into a frittata, following the instructions you'll find in our post on How to Make a Frittata. Let your frittata cool off at least a bit before tucking in. It is also perfectly delicious at room temperature.