Heat oil in a large cast iron skillet or a large nonstick frying pan over medium-high heat. When it shimmers, add peppers and onion and season with salt and freshly ground black pepper. Cook, stirring occasionally, until vegetables are tender and browned, about 6 minutes. Transfer to a medium bowl, stir in cilantro, and let cool slightly, about 5 minutes.
Combine cheeses in a medium bowl and set aside. When pepper mixture is cool enough to handle, scatter 1/2 cup of the cheese mixture over 1 tortilla and spread 1/3 of the pepper mixture over the cheese. Cover peppers with another 1/2 cup cheese and top with a second tortilla. Repeat to make 3 quesadillas.
Wipe out the pan and return it to the stove over medium heat. When it’s heated, add 1 quesadilla and cook until the cheese melts and the bottom tortilla gets a few golden brown spots, about 3 to 4 minutes. Flip and cook for 2 to 3 minutes more. Repeat with remaining quesadillas. Let rest 5 minutes, then cut each quesadilla into 8 pieces and serve.
Beverage pairing: Pacifico, Mexico. Sometimes it’s just the right time and place for a Mexican beer, and quesadillas with peppers, cheese, and onion qualify. Purists might decry dropping a pinch of salt and squeeze of lime into the beer, but those additions make it brighter, more savory, and a more punchy companion to the food.
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