Tostado
Cut two 3-inch rounds from each tortilla. Pan- or deep-fry tortillas until crispy, drain on paper towels, and then season with salt and pepper. Cut the fresh tuna into 2-inch blocks and season with Southwest spice seasoning. Place a nonstick pan over medium-high heat and sear tuna for 1 minute on all sides. Slice tuna thinly and shingle onto the tostados. In a chilled mixing bowl, combine avocado, onion, cilantro, lime juice, chives, black beans, and salt and pepper (to taste). Place each tuna tostada on a plate and top the sliced tuna with a dollop of salsa.
Southwest Spice Rub
Combine all spices in a zip-lock bag. Seal and shake to blend.