Preheat oven to 350ºF. Coat a 10x15-inch jelly roll pan with nonstick cooking spray or brush with 1 tsp canola oil and line it with parchment paper. Pro Tip: gently scrunch the parchment into a ball, and then smooth it out again. Doing this will allow the parchment to lay flatter and sit nicer in the creases and corners of the pan. Allow an overhang of a few inches on all sides, and lightly spray or brush the top of the parchment paper with oil (1 tsp would be sufficient). The parchment makes sure the egg won’t stick to the pan, and the oil on top will ensure the egg doesn’t stick to the parchment when you roll it up. Set aside.
In a skillet over medium heat, add remaining 2 tsp canola oil and sauté onions until just starting to soften, then add the peppers. Sauté for 1-2 minutes, or until desired doneness of veggies. I prefer a little crispness to my peppers, so I don’t tend to cook them too long. Remove from heat and allow to cool slightly.
Place the flour in a small bowl and slowly pour the milk over the flour in a steady stream. Whisk until smooth and ensure there are no lumps.
In a medium bowl, whisk together the eggs, mustard, soy sauce, and pepper. Add the milk mixture and whisk for about 2-3 minutes, or until it has some medium to large sized bubbles. Pour the mixture into the prepared sheet and evenly sprinkle with the sautéed onions and peppers, chopped kielbasa and 1/2 cup of the cheese over the top.
Bake for about 12 minutes, until the edges are set and the center is beginning to set (it will still be a little wobbly). Remove from the oven, sprinkle 1 cup of cheese on top, and bake for another 5 to 6 minutes, until the cheese is just melted and the center has set. Remove the pan from the oven and let cool for about 5 minutes.
Once the omelet and pan have cooled slightly, gently lift the long end of the parchment paper and tightly roll the omelet to create a long cylinder. Sprinkle with the remaining 1/2 cup cheese and the green onions. Slice and serve warm.