Mise en place - Destem and slice the mushrooms. Trim the celery, reserving some of the prettiest celery leaves for garnish. Slice the celery ribs. Trim and slice the scallions, separating the white and green parts. Peel and mince the garlic. Chop the parsley, tender stems and leaves.
Prep the mushrooms - Add the mushrooms to a large, dry pan with a healthy pinch of salt over medium heat. Cover, to sweat the mushrooms, and stir occasionally, as needed. Take off the lid once the mushrooms have expelled all of their water, and keep cooking until the water has cooked off. Remove the mushrooms from the skillet and set aside.
Sauté - Add the olive oil and butter to the pan you used to cook the mushrooms. Turn the heat to medium. Once the oil is shimmering, add the celery and the whites of the scallions. Cook until the celery has softened, then add the broth, herbes de Provence, salt, and pepper. Bring to a boil, then turn off the heat.
Complete the mélange - Add the chicken and mushrooms back to the pan. Stir to combine, then spoon some of the sauce on top. Garnish with fresh parsley and celery leaves, and serve immediately.