Mushroom pasta dough
Blitz the mushrooms in a food processor. Mix the blitzed mushrooms, flour and salt in a bowl. Add the eggs, egg yolk and oil, combine with a risotto spoon. On a lightly floured surface, knead for approx. 15 mins. to form a smooth dough. Shape into a ball, cover, then leave to rest for approx. 30 mins. at room temperature.
Puree the mortadella, Parma ham and mushrooms in a food processor, transfer to a bowl. Mix in the parmesan and sage, season. Knead the filling by hand to form a compact mass, cover and chill.
Quarter the dough. On a well-floured surface, roll out 1 piece into a rectangle until the dough is translucent. Keep separating the dough from the work surface, cut into approx. 5 cm squares. Brush the squares with a little egg white. Divide ¼ of the filling into hazelnut-sized portions and place in the centre of the squares, brush the edges. Fold the dough diagonally over the filling to create triangles. Press the edges together firmly, pressing out any trapped air. Bend the two ends of the long edge inwards and pinch to seal. Place the tortellini on a floured tea towel. Repeat this process with the remaining dough and filling.
To cook the tortellini
Cook the tortellini in batches in simmering salted water for approx. 5 mins. Drain the tortellini, transfer to a bowl, add the butter and sage, mix.