Dice the onion, finely chop the celery, and mince the garlic.

Heat the olive oil and 1 tablespoon of butter in a large skillet over medium-high heat. Add the chicken thighs and sear until golden on both sides, working in batches if needed. Transfer the chicken to a plate.

Add the remaining 1 tablespoon butter to the same skillet. Add the onion and celery and cook for about 5 minutes, stirring occasionally, until softened.

Add the garlic and cook for about 1 minute, just until fragrant.

Pour in the white wine and simmer until reduced by about half, gently scraping up any browned bits from the bottom of the skillet.

Stir in the water or chicken broth, mustard, and Dijon mustard until the sauce is smooth and well combined.

Stir in the heavy cream, then return the chicken thighs to the skillet along with any juices from the plate.

Cover and simmer gently for about 10 minutes, or until the chicken is tender and cooked through.

Sprinkle with fresh parsley and serve hot with plenty of the creamy mustard sauce spooned over the chicken.