In a nonstick skillet coated with cooking spray, saute the onion over medium heat until tender. Add the spinach and cook until wilted, stirring occasionally. Set aside. In a small bowl beat together the eggs and the water, and season with salt and pepper. Coat an 8-inch nonstick skillet with cooking spray, and set over medium-high heat. Pour in the egg mixture. As the egg sets at the edges, gently push the cooked portions with a spatula toward the center. When the eggs are almost set on the surface but still look moist, cover half of the omelette with the spinach filling and sprinkle with the goat cheese. Fold the omelette in half and slide onto a warmed plate.