Adjust rack to top position and preheat oven to 450 degrees. Bring a large pot of salted water to a boil. Wash and dry produce.

Trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Halve, peel, and thinly slice onion. Trim and thinly slice scallions, separating whites from greens.

In a small bowl, combine panko, half the Parmesan, a drizzle of olive oil (large drizzle for 4 servings), salt, and pepper. Set aside.

Heat a drizzle of oil in a large pan over medium-high heat. Add mushrooms; cook, stirring occasionally, until browned and slightly crispy, 5-7 minutes. Season with salt and pepper.

Turn off heat; transfer to a plate. Wash out pan and set aside to cool slightly.

Once water is boiling, add cavatappi to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. 

Reserve ½ cup pasta cooking water (1 cup for 4 servings), then drain.

Heat a large drizzle of oil in pan used for mushrooms over medium-low heat. Add onion and scallion whites; cook, stirring occasionally, until browned and softened, 8-10 minutes.

Reduce heat to low. Add 1 tsp sugar (2 tsp for 4 servings) and a splash of water. Cook, adding additional splashes of water as necessary, until onion is caramelized and jammy, 1-2 minutes more. Season with salt and pepper.

To pan with onion mixture, add mushrooms, cream sauce base, and stock concentrate. Cook, stirring, until simmering, 1-2 minutes.

Stir in cream cheese, cheddar, remaining Parmesan, and 2 TBSP butter (4 TBSP for 4 servings) until melted and combined. TIP: If sauce seems too thick, stir in reserved pasta cooking water a splash at a time until sauce reaches desired consistency. If pasta isn’t finished cooking, ladle pasta water directly from pot.

Season with pepper to taste.

Stir drained cavatappi into pan with sauce.

Transfer pasta and sauce to an 8-by-8-inch baking dish (9-by-13-inch baking dish for 4 servings). Evenly sprinkle with panko mixture. Bake on top rack until panko is golden brown, 2-4 minutes.

Garnish pasta bake with scallion greens. Serve directly from baking dish.