Peal the casing off the turkey sausage and add it to a large bowl. Mash the sausage into small pieces and add the onion, cilantro, mushrooms, cayenne, and crushed red pepper flakes. Stir together until well combined. Heat a large skillet, over high heat and add the sausage mixture. Stir and cook until well browned, about 5 to 6 minutes.
Lightly spray a separate pan with cooking spray and put over high heat. Add 1 tortilla to the pan and put a layer of horseradish Cheddar cheese on half of the tortilla. Top with a layer of the turkey sausage mixture and a thin layer of shredded Cheddar cheese. Add a final layer of horseradish cheese and cover with the other half of the tortilla.
Cook on 1 side for 45 seconds and then flip to the other side and cook for 1 minute. Remove from the pan and cut into 3 triangular pieces. Repeat with the remaining tortillas and filling.
Blend the sour cream and the buffalo sauce in a serving bowl and serve as a dipping sauce for the quesadillas.
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