From: Executive sous chef Michael Reining, Postrio Cafe & Bar, San Francisco. Yield: 10 servings.
1 sheet parchment paper
1 Tbsp. olive oil
2 large onions, thinly sliced
3 cloves garlic, finely minced
⅓ cup Parmesan cheese, grated
1 Tbsp. fresh sage, chopped
1 lb. asparagus, cut into 1" pieces
3 oz. goat cheese, crumbled
¼ cup balsamic vinegar
½ tsp. salt
⅛ tsp. pepper
Preheat oven to 325°F. Prepare a 12"×15" jelly roll pan (sheet pan with sides) by spraying liberally with oil. Place a sheet of parchment paper in the bottom of the pan.
Heat olive oil in large skillet, add onions, salt and pepper. Saute over medium heat for about 10 minutes. Add garlic; continue to cook for about 40 minutes, gently scraping bottom of pan with wooden spoon to keep onions from sticking while caramelizing. Add a bit of water to deglaze if needed. Transfer to a bowl to cool.
In a bowl, beat the eggs. Add onions, Parmesan and sage. In the 12"×15" jelly roll pan, pour egg mixture and spread evenly. Add crumbled goat cheese, cooled onions and asparagus pieces. Cook the frittata in a 325°F preheated oven for 20-25 minutes, until it is golden and firm. In small saucepan over high heat, reduce vinegar to ½ or ⅓ of original volume. Allow to cool.
Loosen frittata gently by running a kinife around edges of the pan. Place a cutting board (big enough to cover entire pan) and flip frittata over and out of the pan. Peel off parchment paper and flip frittata right side up. Brush bottom and sides of frittata with reduced vinegar and cut into wedges. Serve warm or at room temperature.