Preheat oven to 350 degrees.

Heat butter in pan over medium heat. Add onion and cook for 2-3 minutes. Add garlic and cook for an additional 1-2 minutes or until fragrant.

In a small bowl, combine salt, pepper, lemon zest and 1/4 tsp Cajun seasoning. Stir well.

Spray a baking dish with cooking spray. Place one tomato slice in dish. Top with 1/2 of onion mixture. Sprinkle with 1/3 of salt mixture. Top with 1/2 of each cheese.

Add another tomato and each of the onion, salt and cheese layers. Top with tomato slice and sprinkle with remaining salt mixture.

Bake in oven for 10-15 minutes or until cheese is melted.