Snap or cut off the woody ends of the asparagus. It is recommended to peel the lower stalks of medium or fat spears using a vegetable peeler. Cut them into smaller pieces or keep them whole.
In an 11inch (28cm) iron skillet (or any other oven-safe skillet) boil a few inches of salted water and add asparagus. Cook at a rapid simmer for 2 to 4 minutes, depending on their size. Remove with a slotted spoon and set aside. Use paper kitchen towel to remove any remaining water from the skillet.
Crack the eggs into a medium mixing bowl and add the milk and dried mint. Season but use less salt because feta cheese is too salted. Whisk lightly until the mixture is blended and add feta cheese crumbles.
Warm the olive oil over medium heat until shimmering and add garlic and spring onions. Cook for 3-4 minutes, stirring occasionally. Add asparagus for another 2 minutes.
Pour the egg mixture over the vegetables and shake gently to distribute it evenly across the skillet.
When the outside edge and bottom of the frittata are almost done (about 4-5 minutes), transfer the skillet to the oven. Broil for 3-5 minutes (keep an eye on it), until the surface is cooked.
Remove from the oven. Garnish with basil and serve hot.