Prepare a bowl of ice water and have it next to the stove. Bring a large pot of water to a rapid boil. Add a pinch of salt. Add the broccoli florets and cook until crisp-tender, 1 to 1 1/2 minutes. Remove with a slotted spoon and plunge immediately into the ice water.
Place the cooled broccoli and chicken in a mixing bowl and mix with the yogurt, mustard, feta cheese, and lemon juice. Season with salt and pepper to taste. Sprinkle some red pepper flakes if you’d like some heat.
Serve immediately or store in an airtight container in the refrigerator for up to three days.