Preheat oven to 425°F.
Toss raspberries and sugar together in a mixing bowl.
Whisk Cassis and cornstarch together in a small bowl until smooth.
Stir Cassis mixture, lemon juice, and salt gently into berries.
Roll out two thirds of the pastry and line the pie pan; leave edges untrimmed.
Spoon in the berries, dot with butter, and arrange lemon slices overlapping slightly in the center of the berries.
Roll out remaining pastry into a 10-inch round and cut into 1/2 inch strips.
Arrange over berries in a lattice pattern.
Trim overhanging pastry; bring edge of lower crust over lattice and crimp edge decoratively.
Set on the middle rack of the oven and bake at 425°F. for 15 minutes.
Lower heat to 350°F.
and bake for another 30 to 40 minutes, or until crust is golden brown and filling is bubbling.