Thaw the frozen spinach and squeeze dry with paper towels to remove excess moisture.

Drain the canned corn.

In a large microwave-safe bowl, combine water and microwave on high for 7 minutes until hot.

Using a wire whisk or fork, gradually stir in quick-cooking grits, taco seasoning mix, and finely chopped onion into the hot water until well combined.

Microwave the mixture, uncovered, for 4 minutes.

Stir the grits mixture thoroughly and microwave for an additional 2 minutes to ensure they are cooked through and creamy.

Incorporate the drained corn, squeezed spinach, and margarine into the grits mixture.

Add half of the shredded cheddar cheese and half of the shredded Monterey Jack cheese to the mixture.

Stir until the ingredients are well distributed and the cheese begins to melt.

Spray a 13x9 inch baking dish with cooking spray to prevent sticking.

Pour the prepared grits mixture into the greased baking dish, spreading it evenly.

Sprinkle the remaining shredded cheddar cheese and shredded Monterey Jack cheese evenly over the top of the grits mixture.

Bake in a preheated oven at 350 degrees Fahrenheit (175 degrees Celsius) for 30 minutes, or until the cheese is melted, bubbly, and lightly golden brown.

Let cool slightly before serving.