Into a large, deep skillet over medium-high heat, add the spinach leaves. Stir continuously for 3 minutes, or until the spinach is wilted. Remove the cooked spinach from the pan and set it aside. Keep the heat on.
To the skillet, add 2 tablespoons of butter. When the butter is melted, add the onion. Cook for 2 minutes, stirring frequently until softened. Add the garlic and continue cooking for an additional 1 minute.
Add the spinach back into the pan.
Stir in the plant milk then add the remaining 3 tablespoons of butter, stirring frequently until the butter is melted.
Sprinkle in the provolone and Parmesan cheeses. Continue stirring for 2 minutes, or until the cheeses are melted and the sauce is thickened. Remove from heat.
Adjust seasoning to taste with salt and pepper then serve warm.