Step 1: Brown the Ground Beef - Start by browning the ground beef in a skillet over medium heat. Break it up as it cooks until fully browned. Drain any excess grease so the casserole does not become too oily.
Step 2: Add Beef and Onion to the Slow Cooker - Transfer the browned ground beef to the slow cooker. Add the diced onion right on top. The slow cooking process will soften the onion and allow it to blend nicely into the chili base.
Step 3: Stir in the Seasonings and Sauce - Add the salt, black pepper, chili seasoning packets, minced garlic, drained diced tomatoes with green chiles, and tomato sauce. Stir everything together so the beef is fully coated with the chili mixture. Next add half a cup of the shredded cheddar cheese and stir it in. This adds a little creamy richness right into the base.
Step 4: Add the Tater Tot Layer - Spread the frozen tater tots evenly across the top of the chili mixture. Try to cover the surface as evenly as possible so they cook consistently. There is something oddly satisfying about lining up those little tots across the top.
Step 5: Slow Cook the Casserole - Place the lid on the slow cooker and cook on low for four to five hours. During this time the chili mixture thickens and the flavors blend together. The tater tots soften slightly as they absorb the savory sauce underneath.
Step 6: Add the Final Cheese Layer - During the last fifteen minutes of cooking sprinkle the remaining shredded cheddar cheese evenly over the tater tots. Put the lid back on and allow the cheese to melt into a bubbly golden topping.
Step 7: Rest and Serve - Once cooking is complete let the casserole sit for about ten minutes before serving. This allows the sauce to thicken slightly and makes scooping easier. Garnish with sliced scallions if desired and serve warm.