Brown the ground beef and chopped onion in a large pot or Dutch oven over medium heat. Drain off any excess grease.
Add the diced tomatoes with green chilies, tomato sauce, beef broth, taco seasoning, chili powder, cumin, oregano, and garlic powder to the pot.
Bring the mixture to a boil, then reduce heat to low, cover, and simmer for 20 minutes to allow flavors to combine.
If using, stir in the heavy cream during the last 5 minutes of cooking.
Taste the soup and add salt and pepper as needed.
Serve hot with your preferred keto toppings like cheese, sour cream, and avocado.
Notes
For a slow cooker method, brown the beef and onion separately, then combine all ingredients (except heavy cream) in the slow cooker. Cook on low for 6-8 hours or high for 3-4 hours. Stir in cream before serving.
Check your taco seasoning label to confirm it contains no added sugar or starches to keep it strictly keto.