Grease a 6 muffin cups really well or use a silicon tray.
Create bacon nests by lining with bacon, ensuring the base is covered as well. Trim bacon if needed. Optional- Bake off bacon nests in oven for 5 minutes to crispen up if you want crispier bacon.
Beat eggs and mayonnaise in a medium bowl and season with salt and pepper.
Fold in feta, tomato and onion.
Fill bacon nests 3/4 full with egg mixture. Sprinkle with grated cheese
Bake for approximately 18- 20 minutes, until the centre of the muffin is cooked.