Mix all the ingredients for the sauce. Add the chopped tomato, cilantro and onion. Season the tuna very well with salt and pepper. Heat a large skillet over high heat and add the vegetable oil. When the oil is hot, sear the tuna for 30 seconds on each side. Remove from heat and slice the tuna into 1/4-inch slices. To serve, neatly arrange the tuna slices and avocado slices on a plate and drizzle the sauce over the tuna.