The crust of the enormously popular onion tart served at Erwin (2925 N. Halsted St.; 773-528-7200) is actually, says chef-owner Erwin Drechsler, rugelach pastry-minus the sugar and with the addition of whole-wheat flour. “Rugelach is flaky and rich and that’s what we were looking for when we developed this recipe,” he says. At this time of year, you won’t find Vidalia onions, which peak in early summer; Drechsler says any sweet yellow onion, such as Spanish or Walla Walla, will produce excellent results.

Erwin Drechsler’s Caramelized Onion Tart with Blue Cheese, Walnuts, and Sage (Makes 6 large tarts)

For the crust:

• 1/4 cup (1 stick) unsalted butter
• 4 ounces cream cheese
• 1/2 teaspoon salt
• 3/4 cup all-purpose flour, sifted
• 1/4 cup whole wheat flour, sifted
• 1 egg yolk
• 1/2 teaspoon water

(1) Combine butter, cream cheese, salt, and flours and blend with your fingers until mixture resembles a coarse meal with pieces no larger than peas.
(2) Add egg yolk and water, and mix with hands until fully incorporated. Form into a two-inch-diameter log.
(3) Wrap tightly in plastic and refrigerate overnight, or for at least 2 hours.
(4) Preheat the oven to 350 degrees. Cut log into six equal slices and roll them out on a lightly floured surface. The finished rounds should be about 1/4 inch thick and 6 inches across.
(5) Prick each round in several places with a fork. Bake on a parchment- or Silpat-lined cookie sheet pan for 10 minutes, or until light brown.
Cool.

For the topping:

• 4 medium-size sweet onions (Vidalia, Spanish, or Walla Walla)
• 3 cloves garlic, minced
• 2 tablespoons butter
• 1 tablespoon olive oil
• 1 tablespoon sugar
• 1/2 pound blue cheese (Drechsler suggests Maytag or a Danish blue)
• 1/2 cup chopped walnuts
• 1 tablespoon fresh chopped sage

(1) Peel the onions and cut them in half vertically. Place each half cut-side down and slice into 1/4-inch-thick pieces.
(2) In a large skillet, heat butter and oil until it bubbles. Add onions and garlic and cook over medium-low heat, turning frequently, until light brown, about 15 minutes. Add sugar, slightly raise the heat, and continue to cook onions, moving them constantly, until they are a deep golden brown, about 3 minutes.
(3) Allow onions to cool in a single layer, off the heat.
(4) Assemble the tarts: spread a heaping spoonful of caramelized onions over each crust, leaving a 1/2-inch border. Crumble blue cheese over the onions. (At this point, tarts can be refrigerated.)
(5) To serve: Heat oven to 450 degrees. Place tarts on a cookie sheet and bake until the crust is browned and tart is heated through, about 5 minutes. Sprinkle with chopped walnuts and sage. Cut each tart into quarters and serve immediately.

photograph: Tyllie Barbosa