Heat a little oil and a knob of butter in a large oven-safe skillet over medium heat. Add the sliced mushrooms and sauté for 4 to 5 minutes until softened and lightly golden. Remove from the pan and set aside. Add the cherry tomatoes and chopped cooked sausages to the skillet. Sauté for 3 to 4 minutes until the tomatoes begin to soften and release their juices. Transfer to a plate and set aside Add the spinach to the skillet and cook for 1 to 2 minutes until wilted. Remove from the heat and set aside.
In a large mixing bowl, whisk together the eggs, crème fraîche, grated cheese, salt, and black pepper until well combined. Stir the cooked mushrooms, tomatoes, sausage, and spinach into the egg mixture, ensuring everything is evenly distributed.
Wipe out the skillet if needed, then add a little more oil and another knob of butter. Pour the frittata mixture into the skillet and spread it evenly. Cook over a low heat for 3 to 4 minutes, allowing the base to begin setting.
Transfer the skillet to the preheated oven and bake for 10 minutes, or until the frittata is set in the centre and lightly golden on top.Remove from the oven and allow to rest for a few minutes before slicing and serving.