Preheat the oven to 375F. Prepare a baking pan with a sheet of parchment.
Arrange the cheese slices flat and in one layer with room between them (they will melt and spread a little) and bake 5 min, until just melted around the edges.
Let cool 2–3 min, then gently mold into muffin cups. Cool 10–15 min until firm.
While cheese cools, cook ground beef in a skillet over medium heat until no longer pink. Drain.
Fill each cheese cup with beef, lettuce, tomato, sour cream, and green onion. Add other toppings you'd like, such as chopped black olives or a drop or two of hot sauce.