Heat a saucepan on medium heat. Add butter and allow it to melt.Add onion and garlic and saute for a minutes. Add flour and stir continuously till it turns slightly brown.Add vegetable stock in a thin stream, whisking continuously and bring to a boil. Then add rocca leaves.
Let the mixture cook for few minutes. Take it off the heat and let it cool a little.
Blend the mixture in a blender, sieve and adjust seasoning. Serve.