In a food processor, pulse together flour, oregano, and salt. Then pulse in butter until coarse crumbs are formed, leaving some larger pieces of butter. Pulse in water until a cohesive dough is formed.
Turn dough out onto a piece of plastic wrap. Pat into a circle and wrap tightly, then refrigerate for at least 30 minutes.
Remove dough and roll out to fit a 9-inch tart pan. Transfer dough to pan and trim excess. Prick all over and place in freezer for 30 minutes. Meanwhile, preheat oven to 425°F.
Add pie weights or dried beans (with a sheet of parchment between the beans and dough) and bake for 12-15 minutes. Remove and let cool on a wire rack.
Whisk together ricotta, cream, and eggs until smooth and somewhat foamy. Pour into the cooled tart shell and sprinkle Parmesan on top.
Trim spring onions to size of pan, and half lengthwise if they are very large. Arrange on top of the custard.
Bake for 20-25 minutes. Cool 5 minutes in pan, then serve.