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1
Bring a large pot of lightly salted water to a rolling boil. Cook pasta shells at a boil until tender but still slightly firm, about 10 minutes; drain and set aside.
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2
Preheat the oven to 375 degrees F (190 degrees C).
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3
Grease a 9x13-inch casserole dish.
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4
Melt 2 tablespoons margarine in a skillet over medium heat. Cook and stir shallot in melted margarine until lightly browned, about 5 minutes.
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5
Whisk flour into margarine and shallots to form a smooth paste.
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6
Slowly whisk evaporated milk into flour mixture until it forms a smooth sauce; simmer until thickened, 2 to 4 minutes.
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7
Whisk shredded Cheddar cheese and mascarpone cheese into sauce until cheeses have melted, about 5 minutes.
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8
Stir pumpkin puree, lemon zest, salt, black pepper, garlic powder, paprika, nutmeg, and hot sauce into the sauce until combined.
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9
Lightly mix in lobster tail meat, green peas, and cooked pasta shells until heated through, about 5 minutes.
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10
Pour pasta mixture into prepared casserole dish; top with panko crumbs and Parmesan cheese. Drizzle 2 tablespoons of melted margarine over the casserole.
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11
Bake in the preheated oven until topping is brown and crisp, 40 minutes.
