Preheat the oven to 180ºC. Spray a frying pan with low-calorie cooking spray and set on a low heat. Add the leeks and garlic, and gently fry for 5 minutes —until just softened.
Add the sprouts to an ovenproof dish. In a bowl, combine the spreadable cheese, milk, 40g of the Cheddar, Dijon mustard, and garlic granules. Season to taste with salt and pepper. Then mix until smooth.
Add the leeks to the sprouts and mix. Pour over the cheese sauce and mix to coat. Cook in the oven for 20 minutes.
In a small bowl, combine the breadcrumbs, thyme, and remaining Cheddar cheese. Season to taste with salt and pepper.
Sprinkle the breadcrumbs evenly over the sprouts and return to the oven for 10 minutes, until golden brown.