Heat tostada shells at 350 degrees for about 5 minutes. (I placed my on an oiled baking pan, but the oil is not necessary.) In a large skillet, cook 1 pound ground beef (or chicken) with ? cup chopped onion until browned and cooked through. Drain, if needed. Add 2 cups salsa and 1 package taco seasoning mix. Simmer, uncovered, over low heat for about 10 minutes. Now assemble your tostadas: Spread about 1 tablespoon of heated refried beans on each tostada shell. Spoon about ? cup of hot beef (or chicken) mixture on top of that. Sprinkle finely shredded Cheddar cheese on top of the hot beef (or chicken) mixture. Next, top with shredded lettuce and chopped tomatoes, as desired. Place tostadas in a single layer on a large platter to serve. This is *perfect* for a Super Bowl (or other game) party! Enjoy!!!