Quick-pickled onions: toss the slivered onions with lemon juice and salt. Let it sit for 30 mins. This can be made and refrigerated up to 2 weeks in advance.
Chop the muskmelon into bite-sized cubes. Toss with dried cranberries, half the feta (crumble it into chunky bits with your hands), and mint leaves.
Add the pickled onions and half the pickling juices to the salad. Toss it all together and taste. Add more juice if you think it needs it.
Crumble over the rest of the feta and serve cold or at room temperature.