Easy Steakhouse Creamed Spinach

Ingredients

• 4 tablespoons unsalted butter
• 1 medium yellow onion , minced
• 3 cloves garlic , minced
• 4 teaspoons Kosher salt , divided
• 1 teaspoon coarse ground black pepper
• 1/2 teaspoon nutmeg , freshly grated
• 1/4 cup flour
• 3 1/2 cups half-and-half
• 2 1/4 pounds fresh leaf spinach , or one pound frozen
• 1 cup mozzarella cheese
• 4 ounces cream cheese
• 1/2 cup Parmesan cheese , grated