2 bunches broccolini, bottom thirds of stems trimmed
1 rotisserie chicken
2 tbsp. olive oil
4 tbsp. butter
½ lb. white or cremini mushrooms, thinly sliced
1 large shallot or small onion, finely chopped
2 large cloves garlic, chopped
3 tbsp. flour
½ cup white wine or chicken stock
2 ½ cups whole milk
A little freshly grated nutmeg (no more than 1/8 tsp.)
Finely ground white or black pepper
½ cup sour cream or plain Greek yogurt
1 rounded tbsp. Dijon mustard
1 tsp. Worcestershire sauce
3 tbsp. fresh tarragon leaves (stripped from a few sprigs), chopped
8 oz. wide egg noodles
¾ cup panko breadcrumbs
½ cup grated Parmigiano-Reggiano (2 oz.)
8 oz. sharp cheddar, shredded (about 2 cups)
Bring a large pot of water to a boil. Prepare a large bowl of ice water. Salt the boiling water and add the broccolini. Cook until crisp-tender, about 3 minutes. Using tongs, transfer to the ice water. Keep the water at a low boil for the noodles. Drain the broccolini, then chop into bite-size pieces.
Preheat the broiler. Or position a rack in the center of the oven and preheat to 500°.
Pull or chop the chicken meat into bite-size pieces. (Reserve the skin and bones for stock.)
In a large skillet, heat the oil, about two turns of the pan, over medium-high. Add 2 tbsp. butter. When it melts, add the mushrooms and cook, stirring often, until browned, 4 to 5 minutes. Add the shallot and garlic; season with salt. Cook, stirring often, until the shallot and garlic soften, 2 to 3 minutes. Stir in the flour, then the wine. Cook until the wine is absorbed, about 30 seconds. Stir in the milk. Season with nutmeg, salt, and pepper. Stir the sour cream, Dijon, Worcestershire, and tarragon into the sauce, then stir in the chicken and broccolini. Reduce heat to low.
Add the noodles to the pot of boiling water. Cook until almost tender, about 5 minutes. Drain the noodles and transfer to a 3-quart casserole dish.
In a medium bowl, melt the remaining 2 tbsp. butter in the microwave on high. Mix in the panko and Parm.
Top the noodles with the broccolini and chicken. Sprinkle with the cheddar, then the panko. Broil until the top is browned, 3 to 4 minutes.