Preheat the oven to 400F
Season pork tenderloin generously with salt and pepper
Add olive oil to skillet over medium heat. Sear pork tenderloin about 3-4 minutes on each side, or until golden brown. Remove from pan and let rest.
Add the onion, apples, garlic, and thyme to the pan and stir gently.
Place the tenderloin back on top and place skillet in oven. Cook about 10-15 minutes or until internal temperature reads 145F.
Remove the skillet from the oven. Set the apples and pork aside to rest.
Add the chicken stock and bourbon to the pork drippings in the skillet and stir to combine. Bring to a simmer and reduce by about half (about 5 minutes). Then add butter and stir until melted and well combined.
Add the chopped kale to the simmering sauce. Toss the kale in the sauce and stir until wilted, about 2 minutes.
Return the apples and pork tenderloin to the skillet and simmer for another 1-2 minutes. Salt to taste and serve.