Cook the ground beef and onion in a large skillet over medium heat until the meat is browned, breaking the meat into small pieces as it cooks. Drain excess grease.
Stir in Velveeta cheese, Rotel, and taco seasoning, cooking until the cheese melts and is heated through, about 5-6 minutes.
Fill the taco shells with the meat mixture and add toppings such as lettuce, tomatoes, and onions.
Notes
To make in a slow cooker: cook and drain the meat first. Then add everything to the slow cooker and cook on low for 3–4 hours or high for 1–2 hours, stirring occasionally.
Feel free to add extra vegetables like corn or chopped bell peppers to boost flavor and heartiness.
For a dairy-free version, use a meltable dairy-free cheese or omit the cheese entirely. Some alternatives may turn grainy, so choose carefully.
This can be gluten-free, but double-check all ingredients. Some taco seasonings or shells may contain gluten or be processed with cross-contamination risk.