- Preheat oven to 180 degrees. Brush or spray a large lamington pan or similiar with oil. Line bottom of pan with baking paper, leaving a 2cm overhang on the two shorter sides. Brush or spray paper with oil.
- Whisk together milk and flour. Add eggs, mustard and seasoning then whisk to combine. Pour into pan. Sprinkle spinach over top in an even layer. If using frozen spinach then thaw and squeeze out the water and drain on paper towel.
- Bake until edges of omelette are set, 10 to 12 minutes. Sprinkle with cheese; bake until cheese has melted, 2 to 4 minutes. Beginning at one shorter end, lift the paper, and roll up omelette tightly, peeling back paper as you go. Slice and serve.