Spray a 9.5-inch tart pan with a removable bottom with cooking spray. Set aside for the moment.
Preheat oven to 350⁰ F.
Place flour and salt in a mixing bowl. Stir with a whisk. Add cut-up butter and with finger tips or a pastry cutter, cut butter into flour mixture until butter is the size of very small peas.
Sprinkle cider vinegar over and stir in with a fork. Now, stir in ice water, a little at a time, and blend with a fork until mixture can be formed into a ball.
Flour the surface of a pastry cloth or cutting board, and roll dough in a circle, about 11-12 inches in diameter.
Transfer the dough onto the prepared tart pan, and pat down into the bottom of the pan and up along the sides. Trim off any excess dough.
Using a fork, poke holes over the bottom and on the sides of the dough in the tart pan. (If you have pie weights or dry beans, cut a piece of parchment paper the same size as the bottom of the tart pan, and place them on top of the parchment paper. This keeps the crust from bubbling up and getting unshapely. If you don’t have all this stuff, then just bake it with the holes in it and you will probably be good to go. That is what I did for this tart and it worked just fine.)
Bake tart crust for 15-18 minutes. Remove from oven and cool.
TART FILLING DIRECTIONS
Toast nuts in a dry pan over medium-low heat, or in 350⁰ F oven until just beginning to brown and to release their rich, nutty aromas. This takes about 5 minutes using either method. Set aside to cool. (*Be sure to keep your eye on them … they tend to burn easily the second you lose focus!)
In the same skillet, heat olive oil and add red onions. Cook over low heat until caramelized, about 20-25 minutes, stirring occasionally. Depending on the burner, this could take more or less time, but I really like to caramelize the onions for a longer time on lower heat. They turn out perfectly this way!
When onions are caramelized, add sherry and turn heat up to medium. Cook just long enough to deglaze the pan and allow most of the liquid to evaporate. This should only take about 2-3 minutes on medium heat. Remove from stove and cool.
Mix eggs and half & half in a small bowl.
To assemble the tart, place the prepared tart pan on a baking sheet to prevent spills in the oven. Spread onions evenly over the bottom crust. Distribute nuts evenly over that. Sprinkle blue cheese crumbles over nuts. Finally, pour egg mixture over everything.
Bake at 350⁰ F for 30-35 minutes, or until the center is set, slightly puffy, and golden brown.
Allow to sit for 10 minutes before serving. Also delicious served cold.