Finely chop 1/2 onion.
Heat 1 teaspoon olive oil in a frying pan over medium heat and cook the chopped onion until soft and lightly browned; set aside to cool slightly.
Place 1 cup cooked chicken, 2 tablespoons breadcrumbs, 1/4 teaspoon ground sage, 1/4 teaspoon salt, and a pinch of pepper in a food processor and pulse until coarsely chopped.
Add the cooked onion to the processor and pulse to combine.
Taste and adjust seasoning if needed, then add 1 egg and process until the mixture forms a thick paste.
Fill a shallow bowl with a little water. Wet your fingers, take about 1 teaspoon of the mixture, and roll it into a small ball. Place on a plate. Repeat to make about 20 croquettes.
Put 4 tablespoons panko breadcrumbs in a shallow bowl and roll each croquette in the panko until well coated; set aside on the plate.
Heat oil for frying to about 175°C (350°F). Fry the croquettes in batches for about 3 minutes, turning as needed, until golden brown and crisp.
Drain the fried croquettes on paper towels before serving.