Heat olive oil in a large skillet over medium heat. Add ground beef and cook, breaking it apart, until browned. Drain excess grease.
Add chopped onion to the skillet and cook until soft, about 5 minutes. Add minced garlic and cook for 1 minute more until fragrant.
Stir in the taco seasoning mix and water. Bring the mixture to a simmer and cook for 5 to 7 minutes, stirring occasionally, until most of the liquid has evaporated.
Reduce the heat to low. Add the cubed cream cheese to the meat mixture. Stir constantly until the cream cheese melts completely and incorporates, creating a creamy texture.
Remove the skillet from the heat. Stir in the shredded Monterey Jack cheese and sour cream until fully combined and melted into the meat. This forms your creamy filling.
Warm your taco shells according to package directions.
Spoon the cheesy ground beef filling evenly into the taco shells. Top with your desired toppings. Serve immediately.
Notes
For a thicker filling, cook the meat mixture slightly longer after adding the water to reduce excess moisture before adding the cheese.
Use full-fat cream cheese for the best creamy result.
You can substitute shredded cheddar for Monterey Jack cheese if preferred.