1) Preheat oven to 350 degrees F. In a medium bowl, whisk together flour and milk. Crack in eggs and whisk well to combine until the mixture is a single consistency. Season eggs with a big pinch of salt and black pepper.
2) Chop off any large stems from spinach and rinse well. Roughly chop spinach.
3) In a large skillet, add a drizzle of olive oil and all the spinach over medium heat. Season spinach with a small pinch of salt and cook until spinach is wilted, about 2-3 minutes. Remove spinach from heat and let cool slightly.
4) Line a 9×13 baking dish with parchment paper. Press the parchment paper into the corners of the dish and make sure it wraps up the sides a bit. Pour eggs into the dish which should keep the parchment paper in place.
5) Distribute wilted spinach over the top of the eggs and sprinkle with grated cheese. Don’t over-do it on the cheese.
6) Bake omelet for 18-20 minutes, rotating it once halfway through, until the eggs in the center are just set. There shouldn’t be any liquid on top, but try to remove it from the oven before the eggs overcook and crack.
7) Let omelet cool for a minute and then lift the parchment paper out of the baking dish. Fold down parchment paper edges and start rolling omelet.
8) Roll omelet into a tight cylinder and then slice into 8-10 even pieces with a serrated knife. Serve immediately!