Let your raspberries defrost for 5-10 minutes, spread out on a paper towel to absorb some moisture, until they are starting to thaw.
Toss the berries with the white sugar and cornstarch.
Unroll your square of puff pastry straight onto a parchment paper lined baking sheet.
Place the berry mixture in the center.Combine all the ingredients for the streusel.
Fold the edges of the puff pastry into the middle, pinching them together as you fold. You do not have to close the pastry, leave a circle of open raspberry mixture in the middle.
Sprinkle the streusel over the entire galette, pastry included.
Bake in a 400 degree oven for 25-30 minutes. I like to start mine out on the bottom rack of the oven for the first 10 minutes then move to the middle rack for the remainder to make a gorgeous pastry on the bottom and top.