Pre-heat your oven using the grill / broil setting at 450°F/225°C
Break your eggs into a medium-sized bowl and whisk thoroughly. Season with a bit of salt and pepper, and set aside.
Heat a bit of olive oil in a large frying pan over medium-high heat. Add the diced onion and sauté until it's translucent (this should take 3-5 minutes), then add the tomato.
Now add your baby spinach. It may not all fit in the frying pan at first, but have faith. It will wilt down to a fraction of its original size, so just keep moving it around in the pan until it does.
Season the spinach mixture with a bit of salt and pepper.
Pour the beaten eggs over top of the vegetables. Give the pan a bit of a shake to distribute everything
Return the pan to the heat, and allow the frittata to cook over medium heat until the eggs are about half set.
Scatter feta cheese over the top of the eggs.
Put the frittata into the oven under the grill to finish cooking - this should take about 10 minutes - It will be puffed up and golden when it's done.
Let the frittata rest for a few minutes to cool slightly. While this is happening I like to toast my buns under the broiler, but that's an optional step.
Slide the frittata out of the pan and onto a cutting board. Using a biscuit cutter (or, like me, a large coffee cup) cut rounds of frittata that are approximately the same size as your buns.
Place one round of frittata on top of each bun, and then place the lid on top.
Wrap with foil, write the date on them, and toss these bad boys into your freezer.