Spinach Brownies

INGREDIENTS

• 1 (10 ounce) package spinach, rinsed and chopped
• 1 cup all-purpose flour
• 1 teaspoon salt
• 1 teaspoon baking powder
• 2 eggs
• 1 cup milk
• 1/2 cup butter, melted
• 1 onion, chopped
• 1 (8 ounce) package shredded mozzarella cheese