Roll pastry into 12.5" round; cut a silver-dollar-sized hole in the center.
Lay the pastry on a plate, cover, and set in a cool place.
If the peas are at all old, drop them into a big pot of boiling salted water over high heat. When the water returns to a boil, boil for a scant 2 minutes, then drain at once.
Note: the peas and greens mixed with mint can be covered and refrigerated for a few hours - bring to room temperature before assembling the pie.
Heat the oven to 450 degrees F.
In a small skillet, heat 3 tablespoons of the butter and saute the greens and mint over medium-high heat just until wilted, stirring constantly.
Add the wilted greens to the peas, sprinkle on the sugar, salt, and pepper. Taste & adjust seasonings. Mix well, then turn into a 10" pie dish with a rim.
Set 1 tablespoon of the butter in the center, then cut the remaining butter into flakes over the rest of the top.
Lay the pastry on top, tucking the excess underneath itself at the edge of the rim, pinching decoratively.
Bake 15 minutes at 450, then reduce heat to 375 degrees F. Bake until the peas are hot in the center, testing with your fingertip, and the pastry has taken on color, about 15 minutes more.