Heat the oil and butter in a large sauté pan, set over medium-high heat. When butter starts to sizzle, add white onion and cook until soft and translucent, about 3 to 4 minutes. Add the serrano chile and chopped garlic and cook until fragrant, for about a minute.
Incorporate the thinly sliced mushrooms and cook them for about 5 minutes, stirring often. Their juices will begin to come out and after a couple minutes they will begin to dry out. When they do, mix in the epazote leaves if using, and salt, stir and cook for another minute. The mushroom mix should be moist, not wet or too dry, which will be perfect for filling the quesadillas.
Heat the tortillas on a hot comal or dry skillet over medium heat for about 20 seconds. Place a tablespoon or two of the mushroom mix and a tablespoon or two of the shredded cheese (depending on how chubby you want them!) on the center of each tortilla. Fold it as if it were a turnover and press down. Cook for about 2 minutes per side, until cheese is completely melted and tortillas have begun to crisp a bit on the outside.