Preheat oven to 375 degrees Fahrenheit.
Make your ground beef mixture. Heat a large skillet over medium-low heat, add 1/2 tablespoon of oil, and let it get hot. Add onions and saute for 3-4 minutes until softened. Next, add your beef and break it up. Saute for 3-4 minutes. Stir in taco seasoning, tomato paste, and salt and pepper, and mix to combine. Saute for another 3-4 minutes until browned. Remove from the pan and set aside.
When the beef starts to cook, make your tostada shells. Place tortillas in the oven directly on the racks. Bake for 7-10 minutes, flipping halfway through, until crispy and hard. You can also place them on a baking sheet if desired, but I find right on the rack helps with even airflow. Just be careful when flipping and removing them.
When the beef starts to cook, heat up your refried beans. Add your beans to the skillet and break them up. Cook for 5-6 minutes, until warmed through, making sure to stir frequently so they don’t stick.
Assemble your tostadas. Take one tortilla and spread a large heaping spoonful of refried beans on it and spread it out in a thin layer. Then take a spoonful of taco meat and pile it on top of the beans. Top with shredded lettuce, tomatoes, and cheese. Repeat with remaining tortillas, evenly dividing the beans, meat, cheese, and toppings.
Enjoy! Top with more topping if desired and enjoy!