In a large frying pan, heat the oil on medium-high. Add the chicken, skin side down; cook for 10 minutes until the skin is browned. Turn the chicken & cook for 5 minutes until browned; remove from the pan to a plate. Drain all but 1 tablespoon of fat from the frying pan.
Heat the remaining 1 tablespoon of fat & the butter over medium-high heat. Add the mushrooms & cook, stirring occasionally, for 5 minutes or until browned. Add the garlic & cook for 1 minute until fragrant.
Add the salt, pepper & sherry. Cook, uncovered, stirring occasionally, for a few minutes until the sherry has almost reduced by half. Add the broth & mustard; stir to combine.
Add the chicken to the pan, skin side up. Reduce to heat to medium-low & cook, covered, for 20 minutes or until the chicken is cooked – 165°F (74°C) when tested with a meat thermometer. You can turn the chicken, but I like to keep it skin side up during the cooking time.
Add the cream in & around the chicken & stir until combined. Cook, uncovered for 5 minutes on medium heat, or until the sauce is thickened.
Add tarragon & stir to combine. Adjust the seasoning, adding salt and or pepper if needed. If you’re adding the spinach, now is the time. Scatter the spinach into the pan & stir it in around the chicken until just wilted – just a couple of minutes. Sprinkle with extra tarragon leaves. Serve with steamed greens or salad & baby potatoes or steamed rice. Serves 6.