Bring a large pot of salted water to a boil and cook the spaghetti according to package directions for al dente. Drain and set aside.
Melt 2 tablespoons of the butter in a large heavy-bottom pot or Dutch oven over medium-high heat. Add the mushrooms, 1/2 teaspoon salt and a few grinds black pepper and cook, stirring occasionally, until the mushrooms have softened and released their liquid, 5 to 6 minutes. Add the shallots, minced garlic and red pepper flakes and cook, stirring occasionally, until the shallots soften, 3 to 4 minutes.
Add the wine and bring to a boil, stirring to combine. Cook, stirring frequently, until reduced completely, 5 to 6 minutes. Stir in 2 tablespoons of the butter until melted, then sprinkle in the flour and stir until well combined with the vegetables. Slowly pour in the cream, then add the lemon juice, 1 teaspoon salt and a few grinds black pepper. Simmer, stirring occasionally, until the sauce is slightly thickened and no longer has a floury taste, 6 to 8 minutes. Add the pasta and cheese and toss until pasta is well coated. Transfer to a 3-quart casserole dish.
Meanwhile, microwave the remaining 4 tablespoons butter in a small bowl until melted, about 1 minute. Stir 2 tablespoons of the melted butter and 1/2 teaspoon salt into the panko in a small bowl; set aside. Toss the remaining 2 tablespoons melted butter with the shrimp, grated garlic, 1/2 teaspoon salt and a few grinds black pepper in a medium bowl. Scatter the shrimp evenly over top of the pasta, then sprinkle with the panko mixture.
Bake until the shrimp are opaque throughout and the topping is slightly golden, about 10 minutes. Let sit for 10 minutes before sprinkling with the parsley.
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