Heat a small skillet over medium heat. Coat both sides of the corn tortilla with cooking spray. Place the tortilla in the hot skillet, pressing it flat, and cook for 3-4 minutes on each side, until golden brown and crisp. Remove from the skillet and place on a paper towel to drain.
Break 1 egg into a small bowl. Fill a small saucepan with a few inches of water and the vinegar. Once the water is at a simmer, stir the water with the bottom of your slotted spoon clockwise to make a swirling motion in the water.
Quickly and carefully slide the egg into the water. Cover and turn the stove off. Poach until whites are firm and yolks are still runny, 3 minutes and 30-45 seconds.
While the egg is poaching, re-heat the refried beans and steak in the microwave.
Place the cooked tortilla on a plate then slather with the refried beans. Top with steak, cheese, tomatoes, green onions, and avocado slices.
Remove the egg using a slotted spoon and drain a bit over the pan. Add the poached egg to the top of the tostada then season with sea salt and freshly cracked pepper, to taste.
Sprinkle the cilantro over the egg and serve immediately. Enjoy.