To Make Omelettes:
Beat together eggs, water and seasonings. Over medium high heat, melt butter in a 9 inch (23 cm) non stick skillet. Pour 1/2 cup (125 mL) egg mixture into skillet, tilting skillet to cover surface. Cook 2 to 2 1/2 minutes or until set. Slide omelette from skillet. Repeat to make 4 more omelettes.
To Make Cheese Sauce:
In a medium saucepan over medium heat, melt butter. Stir in flour and cook 1 minute. Stir in milk; cook and stir until mixture is thickened. Reduce heat and simmer 2 minutes, stirring occasionally. Season to taste. Stir in 1 cup (250 mL) cheese until smooth. Remove from heat.
To Make Filling:
Press spinach to drain well. In a large skillet over medium heat, melt butter. Add onion, cook and stir 3 minutes. Add spinach and cook over medium high heat until liquid is evaporated. Stir in 1 cup (250 mL) of cheese sauce. Adjust seasoning.
Place one omelette in a greased 9 inch (23 cm) springform pan. Alternate 4 layers of spinach filling and omelette, ending with an omelette on top. Top with remaining sauce and cheese. Cover with foil and refrigerate until ready to bake or overnight. Preheat oven to 375 F (190° C) . Remove foil. Place pan on baking sheet and bake until top is golden and mixture is hot, 35 to 45 minutes. Cover and let stand 15 minutes. Remove sides of pan. Serve in wedges.