Combine ½ cup white vinegar and 1/4 c. sugar in a bowl and whisk till combined along with salt and pepper. Chill thoroughly. Add ¼ cup finely diced watermelon and 1 tbsp finely diced red onion to vinegar and let marinate in fridge for 5 minutes. Meanwhile slice 4 oz sushi grade ahi tuna (saku block) thinly and arrange on serving plate. Drain watermelon and onions and mix with 1 tsp chiffonade mint. Place watermelon mixture on tuna slices and garnish with a healthy drizzle of xvoo, coarse kosher salt and coarse ground black pepper. Serve immediately.