Melt the butter in a large frying pan, add the onion, garlic, thyme and salt and pepper to taste and cook for 5 minutes. Squeeze out all the excess water from the spinach, add to the pan and cook, stirring, for 2–3 minutes until heated through.
Beat together the cream, eggs, cheese and a pinch of salt and pepper in a bowl. Spoon the spinach mixture into the tart case, carefully pour over the cream mixture and bake on a preheated baking sheet in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes until set. Serve with a green salad.