In the bowl of a food processor or in a mortar with a pestle, pulse or pound the parsley, onion, garlic, and capers to a paste.

Add the breadcrumbs and toss or pulse to combine. Slowly add the olive oil, lemon juice, and vinegar in alternating streams, pulsing or pounding well after each addition. Continue mixing until the mixture is a smooth paste. Season to taste with salt and pepper. Serve as a topping over grilled bread or as a dip.