Baked Fish with Lemon Cream Sauce (One Baking Dish!)

Bahan-bahan (4 porsi)

• 4 x 150- 180g / 5 - 6 oz fish fillets (, about 1.5cm / 1/2" thick, skinless and boneless (Note 1))
• 50 g / 4 tbsp unsalted butter
• 1/4 cup cream, heavy / thickened ((See Note 2 for sub options))
• 1 - 2 garlic cloves (, minced)
• 1 tbsp Dijon mustard
• 1 1/2 tbsp lemon juice
• Salt & pepper
• 1 1/2 tbsp eschallots (French onion) (, finely chopped (Note 3))
• Fresh parsley and lemon slices (, to serve)