Prepare mashed potatoes according to package directions. Stir in blue cheese and next 3 ingredients.
Sauté onion in hot oil in a small nonstick skillet over medium-high heat 10 minutes or until tender. Add minced garlic, and sauté 2 minutes; stir into potato mixture.
Unfold 1 pastry sheet on a lightly floured surface; roll out to an 11- x 11-inch square.
Spread half of potato mixture evenly over pastry sheet, leaving a 1-inch border. Roll up, jellyroll fashion, forming a log. Repeat procedure with remaining pastry sheet and potato mixture.
Cut each log into 11 (1-inch-thick) slices with a serrated knife; place slices on a lightly greased aluminum foil-lined baking sheet.
Bake at 400° for 30 minutes or until golden brown. Serve warm.