Add the olive oil to a non-stick skillet over medium-high heat and heat for 1 minute. 

Add the onion and sauté for 2-3 minutes or until translucent. 

Stir in the garlic and taco seasoning. Cook for 1 minute.

Stir in the black beans (including the liquid from the can). Bring to a simmer and mash with back of a wooden spoon until mixture is thick and most of the beans are mashed. 

Stir in the red wine vinegar and salt (if needed).

Toast corn tortillas over gas burners on the stove or in toaster oven. (If you have a gas stove, I just set the tortilla directly on the burner over medium to high heat. Let it cook 30-45 seconds or until charred and flip to repeat on the other side.)

Turn oven broiler on high. Add corn tortillas to a baking sheet and spread with 1/4 of bean mixture on each. Sprinkle cheese on each, dividing evenly between to 4 tostadas. Broil until cheese is melted, 1-2 minutes. 

Make corn salsa by mixing all the ingredients together in a small bowl. 

Add corn salsa to the tostadas and top with more fresh cilantro, chopped avocado, hot sauce or other desired toppings. 

Serve immediately.