Preheat oven to 375°. Spray each of the cups of a mini muffin tin with cooking spray and set aside.
To prepare pie crust, pulse together flour, sugar, and salt until well-mixed. Add in butter and pulse until butter is cut into pea-sized (and smaller) pieces. Pulse in water, 2-3 tablespoons at a time until mixture stays together when squeezed in your hand. Dump mixture onto a well-floured surface, form into two discs, wrap in plastic wrap and stash in the fridge while you prepare the filling.
In a heavy-bottomed dutch oven or skillet, melt butter over medium high heat. Add in red pepper flakes and garlic and cook until garlic is tender and fragrant, about 2 minutes. Add in onion and cook until garlic is translucent, about 4 minutes. Reduce heat to low, add in honey and simmer onions until they turn brown and very soft—about 15 minutes.
Add the mushrooms, spinach, feta, and 1 cup of the cheddar cheese and stir until well-mixed. Remove from heat and set aside.
In a mixing bowl, whisk together the eggs, milk, salt and pepper until pale and bubbly. Set aside.
Remove one of the pie crust discs from the fridge and roll out on a well-floured surface until about 1/8″ thick. Using a biscuit cutter or drinking glass rim, cut out small pie crust circles. Place one circle in each of the cups of the prepared muffin tin—pressing in to form around cup.
To fill each up, put about 1 teaspoon of the spinach mixture in each of the cups, then carefully spoon about 1 tablespoon of the egg mixture over top. Top each mini quiche with a little bit of the remaining cheddar cheese. Repeat until all cups are full.
Bake in preheated oven for 15-20 minutes or until the pie crust is browned and the egg is set. Repeat process with remaining crust and filling.
Serve and enjoy!