1. Season tuna on all sides with salt and pepper. Heat a large frying pan over high heat; add oil. Sear tuna for 45–60 seconds on each side.
2. To make the watermelon salsa: In a bowl, combine watermelon, avocado, shallots, and mint.
3. In a separate container, combine lime juice, olive oil, salt, and pepper and mix well. Pour over the watermelon mixture and let sit for 30–60 minutes before serving.
4. When ready to serve, thinly slice the tuna, arrange on a plate, and spoon some watermelon salsa over it.