Make the filling:
Heat the oil in a skillet.
Add the onions and stir. When they start to brown, add the garlic
Stir for a minute, then add the green peppers and mushrooms.
Saute until the vegetables are cooked. Add the pepper and salt to taste.
Turn off the heat and stir in the coriander. Reserve.
Make the Tortillas:
Place the masa flour in a bowl and add warm water, a little at a time, while mixing with a spatula until a fairly smooth dough forms. Divide into 1 1/2-inch balls. Set aside.
Heat a griddle.
If you have a tortilla press, just follow the instructions to shape the tortilla. If you don't, like me, I sprinkled enough all-purpose flour on my countertop to ensure the dough wouldn't stick, and carefully rolled out the tortillas to about 6 inches in diameter.
Carefully, very carefully, transfer the tortilla to the hot griddle. Flip over when it starts bubbling up and brown spots appear on the underside. Cook the other side.
Make the Mango Salsa:
Mix together all the ingredients and serve with the quesadillas.
To form the quesadillas, heat the griddle and spread 1 tsp canola or other vegetable oil on it.
Now place one tortilla on the griddle, then spread out the mushroom-green-pepper filling on it.
Place another tortilla on top. I also added some guacamole, made with nothing more than avocados, red onions, lemon, garlic powder and salt, to the filling.
When the underside is golden and crispy, flip the quesadilla carefully and cook until the other side is golden and crispy.
Remove to a plate and cut into wedges with a sharp knife or pizza cutter.