In a microwavable mug, add egg, milk, melted butter, salt, and pepper and whisk until thoroughly mixed and egg whites are completely broken up.
Add halved cherry tomatoes, grated cheese, green onion, and half of the chopped chives on top of the egg mixture. Do not mix, you don’t want ingredients to settle to the bottom
Place mug in the microwave, and cook on high for 1 minute, just until egg is completely cooked and quiche is slightly puffed. Garnish with remaining chives and serve immediately.