In a small frying pan over medium heat, sprinkle the feta cheese in a circle, leaving a little space empty in the middle. Crack the egg and pour it into the center of the feta ring. 

Sprinkle the egg with a pinch of salt and pepper and the red pepper flakes.

Place a lid on the frying pan and cook the egg till it has reached your desired doneness - for a runny egg, this will be about 3 to 5 minutes. 

Meanwhile, heat the tortilla in another skillet or over a gas stove to toast it slightly. Remove from the heat and place it on a plate. Mash the avocado across the tortilla in a thin layer. 

Place the feta egg on the tortilla and top with an optional squeeze of lime juice.