Slice the onion and put into a large bowl. Squeeze the lime juice over the onion and let rest. Meanwhile trim the watermelon and cut into bite sized pieces. Remove the onion from the lime juice. Add the honey, salt, pepper and oil to the lime juice and whisk until emulsified. Place the watermelon slices on a platter and top with the onions, herbs and feta. Drizzle the emulsified oil on top of the fruit.