How to Make It
Preheat the broiler. On a rimmed baking sheet, toss the tomatoes, onion and garlic with 2 tablespoons of the oil. Broil 6 inches from the heat for 20 minutes, or until the tomatoes and onion are softened and nicely charred.
In a skillet, toast the árbol chiles over moderately low heat, stirring, until lightly charred, 3 minutes; transfer to a blender. Add the broiled tomatoes, onion and garlic along with the chipotle chile, vinegar and sugar and puree until smooth. Season the salsa roja with salt.
In a cast-iron skillet, heat 1 tablespoon of the oil. Place one tortilla in the skillet and scatter the cheese and zucchini on top evenly. Drizzle over some of the salsa roja and top with the second tortilla. Cook over moderately high heat until crisp on the bottom, 3 minutes. Flip the quesadilla, adding the remaining 1 tablespoon of canola oil, and cook until the cheese is melted, 2 minutes. Slice the quesadilla into wedges, transfer to a plate, garnish with the cilantro and Fresno chiles and serve.