In a large (10-ounce) microwave-safe custard cup or ramekin, combine eggs, milk, cayenne pepper, and salt. Stir in scallion.
Microwave (uncovered) on high for 45 seconds; stir with a fork. Continue cooking until eggs are almost set, about 45 seconds more. Remove from microwave.
With a clean fork, stir in shredded cheddar cheese; cover with a paper (or clean kitchen) towel. Let stand until cheese has melted and eggs are set, about 1 minute. Top with grape or cherry tomato, and serve immediately.