Heat the frozen spinach with water in a saucepan over high heat until thoroughly heated.

Meanwhile, melt butter in a skillet over medium heat and add the minced onion. Cook until translucent, about 10 minutes.

Transfer the heated spinach to a mesh colander and press firmly to remove all excess water.

Add the drained spinach and minced garlic to the skillet with the translucent onions. Sauté for about 1 minute to enhance the garlic flavor.

Pour in the half-and-half and bring the mixture to a low boil.

Stir in the grated Parmesan cheese until incorporated.

Once bubbling again, add the shredded Monterey Jack cheese, stirring until completely melted and smooth.

Season with salt and pepper to taste, then cook for 2 more minutes on a gentle boil.

If the mixture is too thick, add half-and-half one tablespoon at a time until reaching desired consistency.