Spray a non stick skillet with cooking spray. Saute onion over medium heat until tender. Add spinach and cook until wilted, stirring occasionally. Set aside. Beat eggs with water; season with salt and pepper. Spray an 8 inch ( 20 cm) non stick skillet with cooking spray. Heat skillet over medium high heat. Pour in egg mixture. As skillet sets at the edges, with spatula, gently push cooked portions towards centre. When eggs are almost set on surface but still look moist, cover half of the omelette with spinach filling and sprinkle with goat cheese. Fold the omelette in half and slide onto a warm plate.