Into a skillet, add avocado oil, and heat over medium high heat. Brown the chicken, and generously season with salt and pepper.
Flip the chicken to brown the second side, and add balsamic chicken. Reduce the heat to medium low. Simmer the chicken for about 15 minutes, flipping it to ensure even cooking, until the chicken is cooked throughout and the balsamic vinegar is thick and sticky. If necessary, add 1-2 tablespoons of water to prevent the vinegar from burning if it reduces too much.
Onto plates, add chopped romaine lettuce.
Add cubed (seeded) watermelon to each salad.
Top the salads with sliced balsamic chicken.
Garnish the salads with chopped walnuts and mint, and a generous helping of marinated red onions.
Drizzle salads with additional oil from the onion marinade, if desired. A sprinkle of balsamic vinegar is also a great addition!