Heat oven to 200°F for keeping the finished patties warm.
In large skillet, heat 1 tablespoon of the oil over medium heat. Add the apple, onion and pepper flakes; sauté, stirring occasionally, until softened, about 5 minutes. Season with 1/4 teaspoon of the salt. Remove skillet from heat.
Transfer apple-onion mixture to food processor; do not wipe out skillet. Add chicken chunks, black pepper, garlic powder, sage and remaining 3/4 teaspoon salt to food processor. Pulse until all the ingredients are well combined and the chicken is ground but not too broken up, 1 to 2 minutes. Transfer the mixture to a clean bowl.
Wet hands with cold water. Using scant 1/4 cup for each patty, form sausage mixture into twelve patties, 2-1/2 inches in diameter. Place on waxed paper.
In the same large skillet, heat the remaining 1 tablespoon of oil over medium heat. Add 6 patties to the skillet; cook for 4 minutes on each side or until internal temperature registers 160°F on instant-read thermometer. Remove patties to platter; keep warm in 200°F oven (temperature of patties will increase to 165°F as they stand).
Repeat with 6 remaining patties. Serve patties warm.