Spread out the tortillas evenly on a work surface. Divide the cheese and fish evenly, sprinkling the bottom half of each tortilla evenly with the cheese and fish, then with the fresh cilantro.

Fold the tortilla in half to enclose the filling and press firmly. Stack the quesadillas on a plate and set aside.

Brush the pan with oil. Place 2 quesadillas in the pan and cook until the undersides are lightly toasted, about 2 minutes.

Flip the quesadillas and toast the other sides, about 2 minutes more.

Transfer to a baking sheet and keep warm in the oven. Repeat with the remaining quesadillas, brushing the pan with oil between batches.

Cut each quesadilla into wedges and serve immediately with guacamole.