Can I offer just one course meal for the residents because the chef has been ill?

You would still be required to meet the nutritional standards of care, comply with low sodium diets, special diets, diabetic diets, pureed diets, etc.

Have you tried calling Meals on Wheels to deliver for awhile? And if they say "NO", can you hire their chef after hours to prepare foods you can just heat up?

Call a caregiving agency, and ask if they can send a temporary chef/certified nutritionist.

Ask a hospital nutritionist for advice.

A hearty soup, grilled cheese and some fruit/salad?

Should always be a back up plan . Cross train staff so they can step up in these situations.

When the kitchen was undergoing some safety upgrades my mom's nursing home had to have a couple of meals catered.

Not on purpose, no. If you really can't get cover and it really can't be helped, you may get away with this as an emergency. But your residents will have a legitimate complaint if you are not fulfilling the contract they have with you. Can you contact a staffing agency and bring in a temporary chef? How many residents are you dealing with, and what sort of facility are they living in?