Place the minced onion in a medium bowl. Place a mesh strainer over the bowl.

Prepare the citrus. With a sharp knife, slice the off the tops and bottoms. Following the curve of the fruit, slice away the peels and white pith in large, vertical pieces until the exterior of the fruit is free of any white pith. Cut crosswise into 1/4-inch rounds, removing any seeds or stem. Place the rounds and any juices from the cutting board in the strainer over the onion to allow the citrus juice to drain onto the onion. Repeat with remaining citrus fruits.

Arrange the citrus on a serving platter. With a slotted spoon, scoop out the minced onion and sprinkle over the fruit, reserving the juice. Add the vinegar or lemon juice to the bowl of juice and whisk to combine. Whisk in the mustard and olive oil. Season to taste with salt and pepper, then pour the dressing over the salad. Top with feta and mint, and serve immediately.