Heat a small skillet with a teaspoon of olive oil. Toast the sliced almonds for 3-5 minutes, stirring occasionally. Set aside and cool. While cooling, cut the watermelon into chunks and deseed it. Next slice the entire onion in thin slices. Mix the watermelon and onions in a large bowl and top with almonds and feta cheese. Next drizzle balsamic vinegar over salad, sprinkle with mint and toss together gently until mixed. Cover and allowed to stand for 2 hours in the refridgerator for best flavor.