1. Unwrap the phyllo sheets and cover them with a damp towl for 10 minutes. Melt the butter in a saucepan over low heat.
2. Preheat the oven to 4250 F.
3. Using a pastry brush, lightly butter a 14-inch baking pan. Lay a phyllo sheet on the pan. Remember to re-cover the unused phyllo with the damp towel each time. Brush the phyllo with some of the melted butter, and sprinkle with 1 tablespoon of the sugar and 1 teaspoon of the Grand Marnier. Repeat, using 5 more phyllo leaves.
4. Arrange the apples in the center of the top sheet of phyllo in a circular mound about 6 inches in diameter. Brush them with butter and sprinkle with some sugar and Grand Marnier.
5. Stack 6 more phyllo sheets on top of the apples, repeating the buttering and sprinkling with sugar and Grand Marnier. The top (twelfth) sheet of phyllo should only be buttered.
6. Trim off the corners of the phyllo sheets so you have a large round, about 8 inches in diameter. Turn up the edges of the phyllo and pinch lightly to seal. Be tidy, but don't work too long on this; the tart should look rustic.
7. Set the pan on the center rack of the oven and bake until golden, 30 to 40 minutes. If the pastry becomes too brown before this time, cover it loosely with aluminum foil.
8. Serve the tart immediately, or reheat gently before serving.