Heat 1 tablespoon of olive oil in a small skillet over medium heat. Add diced white onion and sauté for about 5 minutes until translucent. Set aside to cool.

In a deep skillet, pour in about 2 inches of oil and heat over medium-high heat to 350°F (175°C). Test oil readiness with breadcrumbs.

In a large bowl, combine sautéed onions, mashed potatoes, minced celery, parsley, sardines, chives, salt, and pepper. Mix until well incorporated.

Shape mixture into small balls or patties about 2 inches in diameter, ensuring they are compact.

Coat each croquette by rolling in flour, dipping in beaten eggs, then rolling in panko breadcrumbs.

Carefully fry croquettes in hot oil for about 30 seconds to 1 minute on each side until golden brown.

Remove fried croquettes with a slotted spoon and drain on paper towels. Garnish with chives and serve with mustard sauce.