Sausage and eggs don't always have to be for breakfast.
Coat a 12-by-17-inch rimmed baking sheet with cooking spray. Place wax paper in bottom; spray generously. Separate eggs, placing whites in mixing bowl. Add salt to egg whites; whisk until stiff peaks form. In a separate bowl, whisk yolks with flour, Parmesan, oregano, and pepper. Add a spoonful of whites to the yolks; mix gently. Using whisk, gently fold in remaining whites.
Transfer mixture to prepared baking sheet; spread evenly. Bake until lightly golden, 10 to 12 minutes. Run a knife around edge to loosen, and turn omelet onto a clean kitchen towel; peel off paper. Gently roll omelet lengthwise, jelly-roll fashion, in towel.
Make filling: Roughly chop sausage; brown in skillet over medium-high heat, breaking it up, about 10 minutes. Stir in tomato. Unroll omelet; layer with half the mozzarella, all the filling, and then remaining mozzarella; roll up, sealing end by brushing it with milk. Place roll, seam side down, on baking sheet; brush with milk. Bake for 10 minutes. Cut into slices, and serve.