Preheat the oven to 400 degrees . In a large nonstick skillet, heat the butter over medium heat until melted, 3 to 4 minutes. Add the onion and cook, stirring, until softened, about 5 minutes. Add the sausage, salt and a few pinches pepper and cook, breaking up the meat with a wooden spoon, until lightly golden, about 8 minutes.
In a large bowl, beat 7 eggs. Add the beaten eggs to the sausage mixture and cook, stirring occasionally, until cooked but not dry, about 5 minutes. Transfer the sausage-egg mixture to a wide shallow bowl. Stir in the cream cheese until just combined. Let cool to room temperature.
On a lightly floured surface, roll out the puff pastry to a 16-inch square. Cut into 4 equal squares. Divide the sausage-egg mixture evenly onto each square, just slightly off center. In a small bowl, beat the remaining egg with 1 tablespoon water. Brush the edges of each pastry square with the egg-water mixture and fold over to form 4 triangles, crimping the edges with the tines of a fork to seal. Brush the turnovers all over the tops and edges with the egg-water mixture.
Transfer the turnovers to a baking sheet and bake until golden, about 25 minutes. Remove from the oven and let cool slightly before serving.