Generously coat tuna steaks with Cajun seasoning.
Heat the oil and butter in a large skillet over high heat.
When the oil is nearly smoking, place steaks in the pan; cook for 3 to 4 minutes or until blackened.
Turn and cook for 3 to 4 minutes, or to the desired level of doneness.
Arrange spinach on dinner plates and top with tuna; sprinkle with lemon juice.