Start by heating a frying pan over medium heat. Add a little oil when pan is hot
Cut the onion in half, root to stem. Thinly slice each half. Add to hot pan with a good pinch of salt. Cook the onion for about 15 minutes. You want to get it caramelized. You'll need to add a little water, about 1 tbsp at a time, occasionally. I like to do this when the pan has some brown bits. Add some water and stir around. The onion will take up the brown bits.
While the onion is cooking, cut the zucchini in half through the middle. Cut each half into half and cut into long slices. You want a similar length to the onion. After about 15 minutes, when the onions are really soft and browning, add the zucchini. Cook for another 5 minutes until all the veggies are super soft.
Take 1 tortilla at a time. Spread a big spoonful of the beans on half, top with some veggies, then cheese. Cook for a few minutes, then flip. Repeat for the remaining two.