Preheat the oven to 375°F.
Place the peas in a colander and run them under room-temperature water for about 30 seconds to defrost. Shake to drain, then leave the colander in the sink for a few minutes to drain further.
Pull about 10 mint leaves from the bunch, and cut them into tiny strips with a sharp knife. Set aside the rest of the bunch – you’ll need it a little later.
Scatter the crumbled feta into the pie crust. Shake the peas in the colander a couple more times, then dump them into the crust on top of the cheese. Tilt and gently shake the pie pan a few times to evenly distribute the peas.
In a medium bowl, whisk together the eggs, milk, flour, salt, and a few grinds of black pepper. Stir in the scallions and minced mint.
Pour the egg mixture carefully over the feta and cheese – filling right up to the top of the crust. (If it doesn’t all fit, don’t force it inches Maybe save it and add to your scrambled eggs for tomorrow’s breakfast.).
Carefully transfer the pie to a baking tray (this is your insurance policy against possible spills) and place the tray in the center of the oven. Bake for about 1 hour, or until the crust is golden, the filling is puffed up and the top surface feels tender but solid when you (carefully!) touch it with a finger tip. If the edges of the crust become too dark before the hour is up, cover the quiche very loosely with aluminum foil to protect it.
Remove the tray from the oven and let the quiche rest for 10 minutes before serving. During this time, cut the remaining mint leaves and use these to top each serving.