Heat oil in a large pan over medium heat and saute onions until soft and translucent. Add spinach, salt, and pepper and briefly saute while stirring constantly, 2 to 3 minutes.. Cover and cook until spinach is wilted, 5 to 7 minutes. Remove lid after 5 minutes and allow cooking liquid to evaporate. Remove from stove and cool completely.
Chop spinach mixture and combine with ricotta and Parmesan cheese in a bowl. Season with salt, pepper, and nutmeg.
Roll out puff pastry sheet and evenly cover with spinach-cheese mixture to within 1/2 inch of the edges. Tightly roll up sheet lengthwise and press edges tightly together. Wrap in plastic wrap and cool in refrigerator for at least 1 hour.
Preheat oven to 400 degrees F (200 degrees C). Cover a baking sheet with parchment paper.
Slice pastry log into 3/4-inch slices so you have 16 to 22 pinwheels. Lay puff pastry pinwheels in the shape of a Christmas tree. The lowest row should consist of 6 pinwheels the second row 5 pinwheels, etc. If you end up with 16 pinwheels, start with 5 pinwheels in the lowest row. Place the last wheel on the bottom to make the stem. Brush with egg.
Bake in preheated oven until wheels are lightly browned and puffed up, 20 to 25 minutes. Decorate with cherry tomatoes for Christmas tree ornaments.