Spanakopita Tart of Caramelized Onion Spinach Mushroom & Feta

The key to this delicious appetizer is to score and bake the puff pastry sheet first.  This raises the edges, leaving room for the filling.  Caramelizing the onions well drives a deep umami flavor, so sweet and rich.
1 13x9-inch puff pastry sheet
3 large sweet onions, thinly sliced
1 Tbsp butter plus 1 Tbsp olive oil

1 lb fresh mushrooms, sliced
1 Tbsp butter plus 1 Tbsp olive oil
1 lb fresh baby spinach leaves
1/4 lb feta cheese, crumbled
1/4 tsp nutmeg
1/4 cup heavy cream

1 egg whisked with 1 Tbsp milk or cream
1 green onion, sliced

Preheat oven to 400F. Thaw the pastry and lay out on an oiled baking sheet.

Caramelize the onions by sautéing in a large skillet in butter and olive oil. Cook over medium low heat, stirring often for about 30-45 minutes - the onions should become nice and brown, sweet and soft.

Meanwhile, sauté the mushrooms in the butter and oil in a large skillet until they give up what water they will and begin to brown, about 10 minutes.

While the mushrooms cook, wilt the spinach in a large pot in a dash of olive oil. Let all the water steam off, or remove with a spoon. Stir in the nutmeg, feta and cream. Remove from heat and keep warm.

Score a rectangle about 1-inch from the border with a sharp knife - don’t cut all the way through, just enough to leave a permanent line. This will make a puffy border for the tart. Prick the area inside the rectangle on the puff pastry with the tines of a fork all over.

Bake pastry for about 10 minutes, just until the edges begin to puff up.

Remove pastry from oven and fill center rectangle with spinach. Top with the mushrooms and the caramelized onion. Brush edges of pastry with the egg mixture. Return to oven and bake another 15 minutes or so, until the tart is nicely browned and everything is heated through.

Remove from oven and garnish with green onion.

Makes one 9x13-inch tart.