Sprinkle the melon with 1 tbsp (15 ml) of the lime juice. Set aside.
In a large non-stick skillet over medium-high heat, soften the onion in the oil. Add the chicken and cook for 2 minutes or until starting to brown. Add the remaining lime juice and the cilantro. Cook for 1 minute. Season with salt and pepper. Transfer to a bowl and keep warm.
In a clean skillet, warm the tortillas in a bit of oil.
Spread the sour cream on the tortillas. Top with the chicken, watermelon, radishes and cilantro leaves. Serve with hot sauce and lime wedges.