Step 1

Preheat oven to 350°F. Generously brush a 2 1/2-to-3-quart baking dish with butter. Char red pepper over a gas flame on high (or, if you don't have a gas stove, on a rimmed baking sheet directly under the broiler) until blackened all over, 6 to 8 minutes.

Step 2

Transfer peppers to a bowl; cover with plastic wrap and let stand 10 minutes. Rub charred skin with a paper towel to remove. Peel, seed, and stem pepper, then cut into 1/2-inch pieces.

Step 3

Cook macaroni in a pot of generously salted boiling water until al dente, about 2 minutes less than package instructions. Drain well; transfer to prepared baking dish. Mix in cubed cheese and 1/2 cup roasted red pepper (reserve remainder for another use).

Step 4

In a large bowl, whisk together flour, 1 1/2 teaspoons salt, mustard, cayenne, and nutmeg. Whisk in sour cream and eggs until smooth, then onion, half and half, cream, and Worcestershire until thoroughly combined. Pour over macaroni mixture; gently shake dish to blend. Sprinkle grated cheese evenly over top.

Step 5

Bake until set around edges but still loose in center, 25 to 30 minutes (do not overbake; mixture will continue setting as it cools). Let cool on a wire rack until set but still warm, 10 to 20 minutes. Serve.