Preheat the oven to Gas Mark 4, 180 °C (350 °F). Butter a 20cm square baking dish.
To cook: Heat a large frying pan over a medium heat, melt the butter. Add the onion and mushrooms and cook for 5 minutes, season with salt and pepper. Add the spinach and thyme and continue cooking until the spinach wilts, stirring constantly. Remove from the heat and allow cool for a couple of minutes.
In a large bowl, whisk the eggs and milk, add ¾ of the cheese and season with salt and pepper.
Transfer the mushroom and spinach mixture into the eggs, stir and pour into the buttered baking dish making sure the mushroom mixture is evenly distributed. Sprinkle over the remaining cheese.
Bake the frittata for about 25-30 minutes or until lightly browned.
To serve: Cut frittata into 4 squares and serve with a salad and some crusty bread.