Thoroughly squeeze all of the excess water out of the spinach using a fine mesh strainer or double thickness of cheesecloth. Place in a large bowl. Add the sun-dried tomatoes, balsamic vinegar and fontina. Season to taste with salt, pepper and a pinch of garlic powder. Combine well and set aside.
Working with one sheet at a time, unfold the pastry onto a lightly-floured flat surface and roll out gently to smooth the creases. Spread half of the filling mixture to within 1/2 inch of the edges of the dough. Roll up as you would a jellyroll and pinch gently to seal the ends. Repeat the process with the other sheet of dough and remaining filling.
Wrap the rolls in plastic wrap and place them in the freezer for 30 minutes to firm up for easier slicing.
Preheat the oven to 400°F.
Remove the rolls from the freezer and slice them into 1/2-inch thick slices. Arrange them in a single layer on a baking sheet and bake for 8 to 10 minutes, or until the pastry is puffed and golden brown. Serve immediately.