With the rack in the middle position, preheat the oven to 200 °C (400 °F). Butter a 33 x 23-cm (13 x 9-inch) or 3 litres (12 cups) capacity baking dish.
In a large pot of salted boiling water, cook the pasta until al dente. Drain and oil lightly. Set aside.
Meanwhile, in a large non-stick skillet, brown the turkey cubes, half at a time, in 30 ml (2 tablespoons) of the butter. Season with salt and pepper. Deglaze with the lemon juice. Set aside in a large bowl.
In the same skillet, brown the mushrooms and onion in 60 ml (1/4 cup) of the butter. Season with salt and pepper. Sprinkle with the flour and cook for 1 minute, stirring constantly. Add the milk and broth and bring to a boil, stirring constantly. Add the peas, 250 ml (1 cup) of Parmesan cheese and the nutmeg. Adjust the seasoning.
Pour the mushroom sauce over the turkey cubes. Add the pasta and stir to combine. Spoon into the baking dish.
In a microwave, melt the remaining butter. In a bowl, combine the bread with the remaining cheese and the melted butter. Sprinkle over the pasta. Bake for about 25 minutes.