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Chop the cooked, cooled lobster meat into bite-size pieces. Refrigerate until needed.
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Shred the cheddar cheese and grate the Gruyère, Havarti, and Gouda cheeses. Combine the cheeses well in a large bowl. Measure out all remaining ingredients. Whisk the flour and cornstarch together in a small bowl. Combine the milk and chicken stock together in bowl or large measuring cup.
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Grease 2-quart casserole or individual serving size oven-safe baking dishes of desired size. Preheat oven to 325°F.
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Cook pasta according to package directions, but undercook it by about 1 minute less than package directions indicate for pasta’s al dente state.
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While pasta is cooking, melt butter in separate saucepan over medium heat. Whisk the flour and cornstarch into butter until mixture starts bubbling, approximately less than 1 minute. Gradually whisk the milk and chicken stock into the butter and flour mixture, stirring constantly, until mixture is heated, smooth, and starts to thicken.
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Add the shredded and grated cheese(s), cream cheese, and sour cream. Stir until cheeses are melted and ingredients fully blended. Reduce heat to medium-low and season with the Old Bay Seasoning, dry mustard powder, sea salt, garlic powder, onion powder, paprika, pepper, nutmeg, and cayenne. Stirring constantly, cook mixture for approximately 1 minute longer. Remove from heat and set aside.
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Drain cooked macaroni, reserving about 3 tablespoons of the cooking water. Return drained macaroni to the pot or a large heatproof bowl and drizzle lightly with olive oil to keep the pasta from sticking together. Toss gently to coat pasta with the oil. Pour cheese sauce over macaroni and stir gently. Fold in the lobster meat and chives. If mixture is a bit drier than, or not as fluid as, desired (different brands of pasta can absorb liquids differently), add liquid reserved from the macaroni cooking, adding the extra liquid only one tablespoon at a time, and adding only enough for the desired consistency (pasta may take some, but not all, of the liquid or none at all). Stir gently.