This is kind of an objective question. I have never seen a recipe which adds coriander to mac and cheese, but that doesn't mean you can't add it. I love coriander to a point where I would eat it poached like vegetables in a Japanese shabu-shabu, so I would definitely try.

If your coriander is fresh, you can do a coarse chop and add towards the end of cooking similar to how you would garnish a dish with parsley. This will prevent the coriander leaves from wilting too much and preserve the freshness and fragrance of the herb.

If your coriander leaves are dry, I would try adding it when you make your roux, or when you are heating the milk/cream/cheese sauce to allow cooking time for the coriander to infuse into the sauce.

If you are American and by coriander you actually mean the coriander seeds and not "cilantro", I recommend the same method as the dry coriander leaves.

This is kind of an subjective question. I have never seen a recipe which adds coriander to mac and cheese, but that doesn't mean you can't add it. I love coriander to a point where I would eat it poached like vegetables in a Japanese shabu-shabu, so I would definitely try.

If your coriander is fresh, you can do a coarse chop and add towards the end of cooking similar to how you would garnish a dish with parsley. This will prevent the coriander leaves from wilting too much and preserve the freshness and fragrance of the herb.

If your coriander leaves are dry, I would try adding it when you make your roux, or when you are heating the milk/cream/cheese sauce to allow cooking time for the coriander to infuse into the sauce.

If you are American and by coriander you actually mean the coriander seeds and not "cilantro", I recommend the same method as the dry coriander leaves.