Add the balsamic vinegar, red wine vinegar, onion, raisins, garlic, anchovies, brown sugar, cumin seed, cloves, and celery seed to a large saucepan and bring to a simmer. Cook until reduced by two-thirds. Strain the mixture, pressing on the solids to get maximum flavor. Discard the solids. The sauce will hold for 1 month in the refrigerator.
Serve with grilled skirt steak garnished with pickled red Fresno chiles, fresh parsley leaves, and sea salt.