Arrange a rack in the middle of the oven and heat the oven to 350ºF. Coat a 9x13-inch baking dish with cooking spray.
Dice onion, mince garlic, and grate parmesan cheese.
Heat olive oil in a Dutch oven over medium heat until shimmering. Add panko and cook, stirring occasionally, until golden-brown, about 3 minutes. Transfer to a small bowl. Wipe out the pot if there are still breadcrumbs inside.
Melt half the butter in the same Dutch oven over medium heat. Add the onion and cook, stirring occasionally, until softened, about 5 minutes. Add the garlic, mushrooms, and 1/3 of the kosher salt. Cook, stirring occasionally, until the mushrooms and onions start to brown, 8 to 10 minutes. Meanwhile, bring a large pot of salted water to a boil.
Add the remaining butter to the Dutch oven. When melted, add the flour, stir to coat the vegetables (it will be a little clumpy), and cook for 1 minute. Add the milk, cream, chicken broth, the remaining 2/3 of the salt, half of the black pepper, and the nutmeg, and stir until no lumps of flour remain. Bring to a boil over high heat.
Reduce the heat to maintain a simmer and cook, stirring occasionally, until the sauce is thickened, about 10 minutes. Meanwhile, chop or shred the cooked chicken.
Add dried linguine to the boiling water and cook for 2 minutes less than the package instructions, about 7 minutes. Drain. Add the pasta, chicken, and frozen peas to the sauce. Stir to combine. Transfer the mixture to the prepared baking dish.
Add the Parmesan cheese and Italian seasoning to the cooled panko, and stir to combine. Scatter evenly over the casserole.
Bake uncovered until lightly browned and the sauce is bubbling, about 25 minutes. Let cool 10 minutes before serving.