Melt butter in a large nonstick skillet over medium heat. Add onion and garlic; saute 5 minutes, stirring. Add spinach, salt and pepper. Cook 2 minutes. Remove from heat; add goat cheese. Let cool.
Unroll 1 sheet puff pastry. Spread with half of filling, leaving a 1/2-inch border on edges. Roll up from a long side. Repeat with second sheet and filling. Wrap both logs in plastic. Freeze 30 minutes. (Can be frozen for up to a month.)
Heat oven to 400 degrees F. Line 2 large baking sheets with foil. Unwrap logs. Cut into 1/2-inch-thick slices and place, cut side down, on prepared baking sheets. (Roll log after 4 or 5 slices to keep pinwheels as round as possible). Bake at 400 degrees F for 20 minutes, rotating pans halfway through (if frozen bake a little longer). Cool on wire rack. Serve warm or at room temp. Makes about 4 dozen pinwheels.