Simmer 3 cups blueberries with 1/2 cup maple syrup until the mixture reduces by half, about 20 minutes. Pour into a bowl; stir in 4 cups blueberries, 1/2 cup maple syrup, 3 tablespoons quick-cooking tapioca, 1 tablespoon,lemon juice, 1/4 teaspoon nutmeg and 1/4 teaspoon salt. Cool completely, then pour into pie shell; dot with 4 tablespoons butter.
Perfect All-Butter Piecrust
Pulse 3 1/2 cups flour, 1/4 cup sugar and 1 teaspoon salt in a food processor. Add 1 diced stick cold butter; process until combined. Add 2 more diced sticks cold butter; pulse three times, or until the mixture resembles coarse meal. Add 1 tablespoon white or cider vinegar. Gradually pour in 1/3 cup ice water, pulsing about four times until combined. Turn out onto a clean surface and press into a cohesive dough without overworking (you should see bits of butter). Wrap in plastic wrap and press into a 1-inch-thick disk; refrigerate at least 1 hour before rolling out.
How to Make a Pie
Divide the chilled dough in half; roll 1 piece into a 12-inch, 1/8-inch-thick circle on a lightly floured surface (refrigerate the other piece).
Roll the dough onto a rolling pin, then unroll it into a 9-inch glass pie plate, letting it hang over the edge; add filling.
Roll out the other piece of dough and place over the filling; press the crust edges together and trim, leaving a 1-inch overhang. Roll up or pinch the overhang to seal.
Place a foil-lined baking sheet on a rack in the lower third of the oven; preheat to 450 degrees. Wrap an oiled, wide band of foil around the pie edge to protect the crust. Make slashes in the top of the crust; chill for 30 minutes.
Reduce the oven to 400 degrees. Bake the pie for 30 minutes. Remove the foil band, brush the crust with heavy cream and sprinkle with sugar. Bake until golden brown, about 30 minutes; cool before slicing.