Defrost pastry sheet and lay flat on a lightly floured surface. Roll with a rolling pin until 1/8 inch thick keeping a rectangular shape. Evenly spread the pesto over the entire surface.
Remove sausage from the casings and crumble into a medium sized frying pan. Brown sausage over a medium heat, continuing to break the sausage into smaller pieces as it browns. Continue browning for 5-7 minutes or until sausage is lightly brown and cooked through.
To carmelize the onions, add 1 tablespoon of olive oil to a medium sized frying pan. Heat oil over medium heat. Add onions and cook for 5-8 minutes or until translucent and golden brown.
Evenly spread the sausage, onions, and cheeses over the pesto.
Roll the pastry starting at the long end of the pastry until fully rolled.
Place the rolled pastry on a greased baking sheet and brush with olive oil.
Bake in a 400 degree oven for 20 minutes or until pastry is puffed and golden brown. Take out of oven and rest for 5 minutes before cutting into 1/4 inch slices if serving as an appetizer or in quarters for an entre serving.