Spray a non-stick skillet with non-stick spray and set over medium-high heat. Add the onion, garlic, cumin and salt. Cook, stirring occasionally, until fragrant, about 2 minutes. Stir in the mushrooms and spinach and cook, stirring frequently until softened, 3 to 5 minutes.
Remove from the heat, add the shrimp and cilantro.
Place a tortilla on a work surface. Top half of the tortilla with 2 tablespoons of cheese, one-fourth of the shrimp mixture, then 2 tablespoons of cheese. Flatten and fold over the top half of the tortilla over the filling to form a semi-circle, lightly pressing the edges to seal.
Repeat with the remaining ingredients, making 4 quesdillas.
Wipe the skillet clean, spray with non-stick spray, and set over medium heat.
Add 2 quesadillas and cook until the cheese melts and outsides of the tortillas are lightly browned, about 4 minutes per side.