Heat butter in a medium pot over medium heat. Add the onion and cook for 3-5 minutes, until tender.
Add the heavy cream, cream cheese, and cheddar cheese. Stir frequently until the cheese is melted and well combined (about 2-4 minutes).
Add the can of drained Rotel tomatoes and chiles, salt, pepper, and cumin. Stir and cook for a minute or two until everything is warm and well combined.
Serve with sliced celery, keto chips, pork rinds, or over chicken/steak. Enjoy!