Easy Cauliflower Gratin With Gruyère (Creamy, Golden, and Simple) - My Home Made Recipe

Ingredients

• 1 large head cauliflower, about 2 lb (900 g), cut into bite-size florets
• 1 1/2 teaspoons kosher salt, divided
• Butter, for greasing the baking dish
• 3 tablespoons (42 g) unsalted butter
• 3 tablespoons all-purpose flour
• 2 cups (480 ml) whole milk, warmed
• 1 teaspoon Dijon mustard
• 1/8 teaspoon ground nutmeg
• 1/4 teaspoon black pepper
• 5 oz (140 g) Gruyère cheese, freshly grated, divided
• 3/4 cup (45 g) panko breadcrumbs
• 1/4 cup (25 g) grated Parmesan cheese
• 1 tablespoon (14 g) unsalted butter, melted
• 1 tablespoon chopped fresh parsley, optional