Lightly whisk the eggs in a bowl and season with salt and pepper.
Cut the mushrooms and red pepper into bite-sized pieces.
Heat a little olive oil in a frying pan and gently sauté the mushrooms and pepper for about 5 minutes, covering with a lid to help them soften.
Add the garlic and oregano and cook for another few seconds.
Stir in the tomatoes and balsamic vinegar and cook until most of the liquid has evaporated.
Add the watercress and let it wilt for a minute.
Pour the egg mixture evenly over the vegetables, reduce the heat, and cook gently for about 8 minutes, until just set.
Sprinkle over the grated cheese, then either cover with a lid or place under a grill until melted and lightly golden.
Slice into wedges and serve warm or at room temperature, garnished with extra watercress if you like.