California Walnut Crostini with Baby Arugula and Caramelized Onions

California Walnut Crostini with Baby Arugula and Caramelized Onions


For the caramelized onion:

  • 1 tablespoon butter
  • 1 teaspoon extra-virgin olive oil
  • 1 medium to large yellow onion, peeled, halved and thinly sliced
  • Salt and pepper

For the seasoned California walnut spread:

  • 1 3-inch chunk of carrot
  • 1 3-inch chunk of celery
  • 1 shallot, peeled and cut in half
  • 1 large bay leaf
  • 2 1/2 cups milk
  • Salt and pepper
  • 2 1/2 cups (8 ounces) toasted California walnut halves and pieces

For the crostini, and serving:

  • About half a sourdough baguette, cut diagonally into slices about ¼-inch thick (for 24 – 30 slices)
  • 1/4 - 1/3 cup extra-virgin olive oil
  • 1 or 2 cloves garlic, peeled
  • 1 1/2 cups, lightly packed, small arugula leaves (about 1 ounce)
  • Salt and pepper


  1. To prepare the caramelized onion, melt the butter in a medium saucepan over medium-low heat. Add the olive oil and onion, season lightly with salt and pepper and stir to combine.
  2. Cook for 20 – 30 minutes, or more, stirring frequently—it is okay if the onions brown lightly, which gives them color and sweetness, but don’t burn them. When done, the onions will have reduced to about 1/4 cup and be a pale golden color. Transfer to a small bowl and set aside.
  3. To prepare the seasoned walnut spread, wrap the carrot, celery, shallot and bay leaf in a double thickness of cheesecloth and tie the bundle securely with twine. Put in a medium saucepan, pour in the milk and season with 1/4 teaspoon salt and 1/8 teaspoon pepper. Bring to a simmer over medium-high heat. Add the walnuts, reduce the heat, and poach in the simmering milk for 18 – 20 minutes, stirring once or twice.
  4. Strain, reserving the milk. (Discard the cheesecloth bundle.) While the walnuts are still warm, put them in a food processor. Add 1/3 cup of the reserved milk and puree. Add additional milk by tablespoons, until the mixture is smooth and spreadable. Season to taste with salt and pepper, transfer to a bowl and set aside. (Any remaining milk may be used in a soup or sauce.)
  5. To prepare the crostini, brush the baguette slices lightly with olive oil, put them in a single layer on a baking sheet, and place in a preheated 350ºF oven for about 5 minutes, to dry and crisp. Remove from the oven and gently rub each slice with garlic.
  6. Spread about 1 tablespoon of seasoned walnut mixture on each crostini. In a small bowl toss the arugula with 1 tablespoon olive oil, then season lightly with salt and pepper. Drape a few leaves of arugula over each crostini and top with about 1/2 teaspoon caramelized onions.