Heat 2 tablespoons of olive oil in a large cast-iron skillet over medium-high heat. Add the turkey bacon and onion and sauté until the latter is caramelized, about six minutes. Using a slotted spoon, transfer bacon and onions to a large bowl.
Place Brussels sprouts cut-side down in the same skillet, spreading them out in a single layer. Brown well on one side before stirring, about six minutes. Add salt and pepper; stir and cook until soft on the outside but still firm inside, about six more minutes; transfer sprouts to the bowl with the bacon and onions.
Add apples to the same hot pan and brown for three minutes on each side, then add the chicken stock and vinegar to the pan; reduce heat to low. Add all ingredients except sage back into pan, and cook for two to three minutes. Remove from heat, transfer to a bowl, and sprinkle with chopped sage.
Photography and styling by Julia Gartland