Heat 1 tablespoon of olive oil in a medium skillet over medium heat.

Add the thinly sliced onion to the skillet and stir to coat with oil.

Cook the onion, stirring, until softened, about 3 to 4 minutes.

Reduce the heat to low and continue to cook, stirring occasionally, until the onion is golden brown, about 7 to 8 minutes more.

Remove the onion from the heat and let it cool.

In a food processor, combine the cooled onion, parsley, salt, red-wine vinegar, cilantro, oregano, lemon juice, garlic, pepper, cumin, cayenne, and 5 tablespoons of olive oil.

Process the mixture until the ingredients are finely chopped and come together, about 15 seconds.

Set the chimichurri sauce aside.

Preheat a grill to medium-high heat.

Brush the cut sides of the ciabatta bread with the remaining 1/4 cup of olive oil.

Place the bread on the grill, cut side down, and grill without turning until the underside is lightly charred, about 4 minutes.

Cut each bread half crosswise into 3 pieces.

Spread each piece of grilled bread generously with the prepared chimichurri sauce and serve immediately.