Tosta de Boquerones

To make the pickled onions, in a small saucepan over medium heat, combine 1 1/2 cups (12 fl. oz./375 ml) water, the vinegars, garlic, serrano chile, dried chile, peppercorns, sugar and salt.

Bring to a simmer, stirring occasionally, until the sugar and salt are dissolved. Place the onion in a heatproof jar or bowl. Pour the hot vinegar mixture over the onion so that all the slices are submerged. Let cool to room temperature, cover and refrigerate for at least 1 hour and up to 1 week.

In a blender or mini food processor, combine the egg yolk, mustard, lemon juice, pimentón and salt and process to combine. With the motor running, slowly add the olive oil, just a few drops at first and then in a steady stream, until the oil is completely incorporated and the mixture is thickened. Transfer to a bowl and set aside.

Cut the bread into slices about 3/4 inch (2 cm) thick, then cut each slice in half. Line a baking sheet with paper towels.

In a large fry pan over medium heat, melt 2 Tbs. of the butter with 2 Tbs. of the olive oil. Working in batches, fry the bread, turning once, until golden brown and toasted all over, about 3 minutes per side, transferring the slices to the prepared baking sheet as they are finished. Repeat with the remaining bread slices, adding the remaining 2 Tbs. butter and 2 Tbs. olive oil to the pan as needed.

To assemble, spread each piece of toast with a spoonful of the aioli. Top each piece of toast with a boquerón and a few pieces of pickled onion. Sprinkle very lightly with pimentón and serve immediately. Serves 6.

Williams-Sonoma Test Kitchen