Preheat the oven to 180°C (350°F). Place the tortillas on a baking tray in a single layer. Bake for 20 minutes, or until crisp and golden brown. Remove from the oven and set aside.
Heat the oil in a saucepan over medium heat. Add the chopped onion, chopped tomato, black garlic salt, and ground cumin. Sauté for about 5 minutes, until softened and fragrant.
Add the drained black beans to the saucepan and cook until they start bubbling. Turn off the heat and mash the beans with a potato masher until smooth but still slightly chunky.
Crumble the feta cheese and slice the avocado.
Spread a generous layer of mashed beans over each baked tortilla. Top with feta crumbles, avocado slices, and your favourite spicy salsa if desired.
Arrange the tostadas on a platter and serve immediately. Perfect for a quick, healthy lunch or sharing with family.