Heat up the skillet or tawa on low heat.
Place one tortilla shell on a plate and sprinkle cheese all over it by hand. Cover it evenly and try to get cheese on the entire shell.
Sprinkle in taco seasoning, onions, mushrooms, and anything else you are adding.
Carefully holding on to the tortilla by two side, transfer it onto the skillet on the stove. Be careful not to touch the skillet. Rest it flat onto the skillet, being careful not to let anything fall off into the frying pan. (you can make this easier for yourself if you start with the tortilla shell on the frying pan and add your ingredients there, but the shell won’t heat up as evenly as it should this way.)
Place the other tortilla shell on top of the first one, pressing down slightly with your hands or a spatula. Be sure to press down across the entire quesadilla. One of the tricks to how to make cheese quesadillas on stove is to ensure that your two shells are sealed together. Pressing them down helps the cheese to melt onto both the top and bottom. You should keep pressing every so often just to ensure the two sides stick together.
Continue cooking on low heat until the bottom tortilla is crispy and the cheese has begun to melt. If it has melted enough, you will be able to flip the tortilla over without anything falling out. To flip your tortilla, you can use two spatulas or two sets of tongs. You want to grab the tortilla with your utensils on two opposite ends and then flip it gently but quickly over to the other side, placing it in the middle of your skillet. If you are skilled, you can flip it over with one spatula, but this increases the risk that something will fall out.
The quesadilla needs to cook on this side as well, so cook it for a few minutes until it browns. Once both sides are brown, you can take the quesadilla off the stove and place it on a cutting board.
Now, it is time to cut the quesadilla into quarter sections. You can cut it in smaller pieces, if you like. Start from one end with your knife and cut a straight line to the opposite end. Then, find the halfway mark between two of the cuts and do the same thing, cutting from one end to the opposite one. You now should have four equal portions of quesadilla.
Serve with pico de gallo, sour cream or salsa for dipping.