Preheat oven to 400°F (200C). Grease baking dish; set aside.
In large pot of boiling salted water, cook squash, carrots and garlic cloves for 8 to 10 minutes or until tender. Meanwhile, cook macaroni in separate pot of boiling salted water according to package directions; drain, reserving 1/2 cup (125 mL) cooking liquid.
In food processor or blender, purée cooked vegetables, reserved cooking liquid, milk, oil, salt, pepper, mustard powder and nutmeg until smooth. Stir in 2/3 of the Cheddar cheese and 1/2 of the Parmesan cheese.
Toss pasta with vegetable sauce; spread evenly in prepared pan. Sprinkle with remaining Cheddar and Parmesan cheese. Bake for 15 to 20 minutes or until golden brown and bubbling.
Use aged, medium or mild Cheddar cheese, depending on preference.Garnish with finely chopped fresh parsley or green onions if desired.