Creamy leeks with Shropshire Blue & tarragon on toast

Name: Creamy leeks with Shropshire Blue & tarragon on toast
Prep time: 5 min
Cook time: 15 min
Total time: 20 min
Cuisine: British
Ingredients: 1 medium leek, trimmed; 10 g salted butter; 1 slice/s thick slice sourdough; 1 sprig/s tarragon, leaves only, finely chopped; 2 tbsp double cream; ½ tsp Dijon mustard; 30 g Long Clawson Shropshire Blue (or a strong Cheddar or your favourite hard cheese), grated or finely chopped; 1 lemon, juice plus 1 wedge; Parmigiano Reggiano, finely grated (optional)