Heat a skillet over medium heat. Melt butter, add shrimp; sprinkle with Cajun seasoning. Sauté for 2 minutes, flip shrimp and sauté another 1-2 minutes until shrimp have turned pink. Remove from pan and reserve.
Add 1 Tbl. butter to skillet, add spinach and sauté for 2 minutes, stirring occasionally. Add garlic and sauté for an additional minute. Remove and drain on paper towels.
Melt butter in pan over medium heat; add flour tortilla. Top with shredded cheese, sautéed spinach, shrimp, and a bit more cheese. Top with second tortilla; flip when golden brown. Pan fry for an additional minute until golden brown. Remove from pan and cut into quarters or sixths. Repeat for second quesadilla.