Heat the oil over medium heat and stir-fry the mushrooms and thyme for 4-5 minutes. Stir in the lemon zest and juice for 1 minute.

Preheat the oven and reduce the heat of the pan.

Beat the eggs and season with salt and pepper. Add the watercress. Cook the mixture together with the mushrooms for 7 minutes.

Transfer the mixture to the oven. Bake for 4-5 minutes.