To a large skillet, add the oil, onions, and cook over medium-high heat for about 7 minutes, or until onions begin to soften and caramelize; stir frequently.
Add the chicken, season with salt and pepper, and cook for about 4 to 5 minutes, flipping and stirring intermittently so all sides cook evenly. Chicken should be about 80 percent cooked through.
Add the garlic and cook for about 1 minute, or until fragrant; stir frequently.
To a medium bowl or large measuring cup, add the apple cider, Dijon, whisk to combine, and add to the skillet.
Turn the heat to low, cover, and simmer for about 7 to 10 minutes, or until chicken is cooked through. While the chicken simmers, make the apples.
To a separate large skillet, add the butter and heat over medium-high heat to melt.
Add the apples, brown sugar, cinnamon, lemon juice, stir to combine, and cook for about 5 to 7 minutes, or until apples are just tender. Stir frequently to prevent burning.
Taste the chicken and cider-Dijon mixture, add additional salt or pepper to taste, if desired.
Evenly sprinkle with sage and serve immediately.
Notes
*Apple cider: In the US, apple cider is a still, non-alcoholic beverage. In other parts of the world, it may be called mulled cider. Do NOT use alcoholic hard cider in this recipe. Storage: Dish is best warm and fresh but will keep airtight in the fridge for up to 5 days. Store chicken and apples in separate containers. Reheat gently as desired.