Macaroni and Cheese with Cauliflower

Preheat the oven to 400 degrees F (200 degrees C).

Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Add cauliflower during the last 3 minutes of cooking. Drain and transfer to a bowl.

Reduce heat to medium and add oil to the pasta pot. Add onion, salt, and pepper; cook, stirring occasionally, just until soft, 5 to 7 minutes. Mix in pasta-cauliflower mixture, Cheddar cheese, sour cream, milk, and Dijon mustard. Transfer mixture to a shallow 3-quart baking dish. Sprinkle with bread crumbs.

Bake in the preheated oven until golden brown, 12 to 15 minutes. Top with parsley before serving.