Pre-heat your grill to medium. Heat the olive oil in a large ovenproof frying pan. Add the watercress and gently fry until completely wilted, about 7-10 minutes.
In a separate bowl, gently mix your eggs with a fork and add a splash of milk and combine. Season well. When the watercress is wilted, add to the egg mixture and stir to coat evenly.
Pour the egg and watercress mixture back into the frying pan and move around to get an even coating. Gently work the egg away from the bottom in a few places and then let the uncooked egg fill the space. Once the bottom of the Frittata is set, around 4-5 minutes, place the pan under the grill to set the top of the Frittata.
Once set, about 4 minutes, gently slide the Frittata out onto a plate and serve in wedges with a crisp salad.