In a cast iron skillet or large saute pan, heat olive oil over medium heat. When oil is hot, add sliced onions, season with salt, and saute until caramelized (approx 20-30 minutes).
Put pie crust in 8-9 inch pie pan or tart pan. Bake for 6 minutes (you may want to put parchment paper and rice or dry beans over the crust to keep it from bubbling up).
In a medium bowl whisk eggs, heavy cream, sour cream, tarragon and salt and pepper until combined.
Remove pie crust and discard parchment paper (if used). Spread crumbled blue cheese on the bottom of pie crust. Cover the crumbled cheese with the caramelized onions and spread evenly to cover the bottom of the crust. Poor egg and cream mixture over the onions and cheese.
Bake for 35-40 minutes until golden brown and set in the middle. The filling may rise. Allow to cool slightly and settle in crust before serving.