For the phyllo crust:
8 14" x 9" store-bought phyllo dough sheets, defrosted (* see cook's note below)
2 tablespoons unsalted butter, melted
2 tablespoons olive oil
For the caramelized onions:
1 large yellow or sweet onion, peeled, ends removed and thinly sliced
1 tablespoon unsalted butter
1 tablespoon olive oil
Salt, to taste
For the filling:
3-4 medium beets, ends removed, peeled, halved and sliced into 1/8" slices
6 ounces Feta cheese (Blue cheese or Stilton also works well in this recipe if you're looking for a more pungent taste)
4 ounces pine nuts or chopped walnuts, toasted
Handful of freshly chopped Italian parsley
Honey, for drizzling
Make the caramelized onions: Heat butter and oil in a large, heavy skillet to medium on the stove top. Add onions and sauté, stirring every few minutes, until the onions are a golden brown. The entire process should take about 25-30 minutes. Season lightly with salt the last 5 minutes of cooking. Once caramelized, remove onions from the pan and set aside to cool.
Preheat oven to 375 F and make sure your oven rack is in the middle of the oven.
Combine melted butter and oil in a small bowl. Place a large, nonstick baking sheet on your work surface. Take one sheet of phyllo and lay flat on the baking sheet. Brush lightly with the oil mixture. Repeat until all phyllo sheets have been used; brush the top layer with the oil mixture.
Layer caramelized onions on top of the phyllo and spread out evenly with the back of a spoon, leaving a 1" border around the edge of the phyllo sheets with no onion. Layer beet slices on top and brush lightly with the remaining oil mixture. Bake for 30-35 minutes, until the beet slices are fork-tender and beginning to brown and the edges of the pastry have puffed up. Remove from oven and let cool for a few minutes.
To finish the tart, crumble cheese in an even layer over the beets. Sprinkle pine nuts and parsley over the top. Drizzle with honey. Cut into slices with a sharp knife and serve.
* Cook's Note: When working with phyllo dough, it's important not to let the dough dry out as you're working with it. Token the dough moist (but not wet), dampen a couple of clean kitchen towels with water. Lay one towel on your work surface, place the defrosted phyllo, unfolded on top of the damp towel and cover with the other towel. As you pull one sheet of dough out of the stack make sure to recover with the top damp towel until all phyllo sheets are used.