1½ lbs. onions, peeled and quartered1½ Tbsp. olive oil 2 Tbsp. sherry vinegar Salt and freshly milled pepper ½ tsp. ground cloves 6 eggs 2 Tbsp. chopped parsley 2 Tbsp. butter ½ cup walnuts, roasted
Slice the onions crosswise about ¼ inch wide. Warm the olive oil in a ten inch skillet, add the onions and cook over medium heat, stirring occasionally, until golden and soft, about 30 minutes. Add half the vinegar, let it reduce, then season well with ¾ teaspoon salt, pepper to taste, and the cloves. Preheat the broiler. Whisk the eggs, season them with a few pinches of salt, then add the onions and parsley. Melt a tablespoon of butter in skillet until it’s sizzling, then add the eggs and lower the heat. Scatter the walnuts on top and cook until eggs are set and nicely browned on the bottom, 8 to 10 minutes. Slide the pan 4 to 6 inches under the broiler to finish cooking the top, but take care not to burn the walnuts. Loosen the frittata and tilt onto a serving platter. Return the pan to the stove and raise the heat. Add the remaining butter and, when it begins to foam, add the remaining vinegar. Slide the pan back and forth to combine the two, then pour it over the eggs.
from Vegetarian Cooking for Everyone by Deborah Madison