Method

• Heat the oven to 375°F (191°C). Line the 15 x 10 x 1-inch (38 x 25 x 2.5 cm) jelly roll pan with parchment paper, allowing a little overhang on the short sides so the baked egg sheet releases cleanly.
• Heat the olive oil in the 10-inch (25 cm) skillet over medium-high heat. Add the bell pepper and mushrooms and cook for 4 to 5 minutes, stirring occasionally, until softened and the skillet looks nearly dry; evaporation now prevents a watery filling.
• Add the spinach and half of the scallions to the skillet. Cook for 1 to 2 minutes, stirring, until the spinach has collapsed and visible moisture has evaporated, then remove the skillet from the heat.
• Whisk the eggs in the large mixing bowl until the whites and yolks are completely blended. In the small bowl, whisk the flour with the milk until smooth, then whisk it into the eggs with the salt and black pepper until evenly combined.
• Pour the egg mixture evenly into the prepared pan and bake at 375°F (191°C) for 13 to 15 minutes. The surface should look set with no liquid egg remaining, and the center should reach 160°F (71°C); controlled oven heat sets the proteins evenly without excessive browning.
• Remove the pan from the oven and evenly scatter the cooked vegetables, remaining scallions, and cheddar over the egg sheet, leaving about a 1-inch (2.5 cm) border. Return it to the 375°F (191°C) oven for 2 to 3 minutes, until the cheese has melted.
• Remove the pan from the oven and immediately loosen any exposed edges with the silicone spatula. Starting at one short end, use the parchment to help roll the warm omelette firmly but gently, peeling the parchment away as you move; rolling before the egg cools preserves flexibility.
• Place the omelette roll seam-side down and let it rest for 5 minutes so residual heat finishes melting the cheese and the egg structure settles. Cut into about 8 slices and serve while warm.