Cheesy leeks

Name: Cheesy leeks
Prep time: 20 min
Cook time: 20 min
Total time: 40 min
Ingredients: 600 g Leeks; 2 tsp Thyme, leaves; 1 tsp English mustard; 50 g Plain flour; 50 g Stale bread blitzed to crumbs; 50 g Butter; 100 g Mature cheddar, grated; 500 ml Milk