Cheesy Flounder Florentine

Ingredients (12)

• ✓ 1 ½ cups milk
• + 4 (5 ounce) flounder fillets add to pantry
• + 8 ounces spinach fettuccine add to pantry
• + ¼ cup butter, divided add to pantry
• + ¼ cup all-purpose flour add to pantry
• + 1 ½ cups chicken stock add to pantry
• + 1 teaspoon Dijon mustard add to pantry
• + ¼ teaspoon ground black pepper add to pantry
• + ¼ teaspoon ground nutmeg add to pantry
• + ⅓ cup grated Parmesan cheese, divided add to pantry
• + ⅓ cup shredded Gruyere cheese, divided add to pantry
• + 1 (10 ounce) package frozen chopped spinach, thawed and drained add to pantry