Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch casserole dish with cooking spray.

Heat a large skillet over medium-high heat and stir in ground beef, green onion, and 1/2 package taco seasoning. Cook and stir mixture until the beef is crumbly, evenly browned, and no longer pink, about 10 minutes. Drain and discard any excess grease.

Combine remaining taco seasoning mix with tomato soup, nacho cheese soup, and milk in a saucepan; heat soup mixture until just boiling.

Layer potato nuggets in the prepared casserole dish.

Bake in the preheated oven until potato nuggets begin to crisp, about 10 minutes.

Spoon ground beef mixture over potato nuggets; pour in soup mixture.

Bake until soup mixture is bubbling, 15 to 20 minutes. Sprinkle casserole with Mexican cheese blend; bake until cheese is melted, 5 to 10 minutes.