Heat 1 Tbsp. oil in a medium skillet, add onions and cook over medium heat until translucent, about 2 to 3 min. Add apples and cook until just tender, another 2 to 3 min. Remove from heat and transfer to a medium bowl. Add remaining ingredients and mix well with your hands. Refrigerate up to overnight, if desired, to let flavors incorporate. Form 2 Tbsp. of mixture into a ball, then press into a patty. Repeat with remaining mixture. Heat another 1 Tbsp. oil in a medium skillet. In batches, cook patties over medium-low heat, covered, until cooked through, about 10 min. total. Turn patties over every few minutes to ensure even browning. Add additional oil to the skillet between batches if necessary.
These make a great side to breakfast omelets or pancakes.