¼cupfresh basilcut into chiffonade (that's thin slivers)
In a large bowl, cream together butter, sour cream and olive oil with a stand or hand mixer.
Then add in flour, salt and Parmesan cheese. Mix together just until combined. Dough should be sticky, but if it is very sticky that you cannot even handle it, then add a touch more flour.
Divide the dough into two balls, and flatten into disks. Wrap each one in plastic wrap and refrigerate for at least two hours or overnight. This will make it much easier to handle.
When ready to bake, roll out each ball one at a time on a lightly floured surface to about 9 inch in diameter circle about 1/8 inch thick. Keep the other ball in the refrigerator until you are ready to use it.
Sprinkle the mozzarella cheese, tomatoes and basil evenly on the surface, not getting too close to the edge.
Cut the circle into 12 triangles with a sharp knife or pizza cutter and roll up each triangle from the wide end, and secure the tip onto the bottom so you have a spiral. Place the rugelach on a cookie sheet lined with parchment paper. Repeat with other dough.
Refrigerate rugelach for 30 minutes and preheat oven to 350 F.
Combine egg with 1 tablespoon of water and wash the rugelach. Sprinkle with a little salt.
Bake rugelach for 25 minutes or until cheese is bubbly and dough is golden brown. Best warm right out of the oven! You can bake them ahead of time and reheat in the oven, or form a few hours ahead of time and bake right before serving.
You'll also need at least two hours to chill the dough.