Heat a large skillet over medium heat. Add 1 lb ground beef and 1 yellow onion (chopped). Brown the beef, breaking it up with a spoon, until the beef is cooked through and the onion is softened and translucent, about 5–7 minutes. Drain and discard excess fat.

Return the skillet with the beef to medium heat. Add 1/2 cup green chili enchilada sauce, 1 tbsp taco seasoning, 1 tbsp lime juice, 1/2 tsp salt, and 1/4 tsp cayenne pepper. Stir to combine and cook 2–3 minutes more, until the sauce has thickened. Remove from heat.

Spoon the contents of the 1 16-oz can refried beans into a small microwave-safe bowl. Cover with a damp paper towel or microwave-safe lid. Microwave in 30–45 second intervals, stirring between intervals, until heated through (about 1–2 minutes total). Alternatively, warm the beans in a small saucepan over low heat, stirring until heated.

Arrange 4–6 tostada shells on a work surface or plates (use the number of shells you have).

Spread a spoonful (or your preferred amount) of the warm refried beans over each tostada, covering most of the surface.

Divide the seasoned ground beef evenly among the tostadas and spread it over the beans.

Top each tostada with any of the optional garnishes as desired: shredded lettuce, salsa, avocado shells, Mexican cheese, Mexican crema, and chopped cilantro.

Plate and serve immediately.