Vegetable bake

Name: Vegetable bake
Ingredients: 1 celeriac, peeled and cut into cubes (about 950g); 1 cauliflower, broken into florets, stem finely chopped, leaves reserved; 3 garlic cloves, unpeeled and bashed; 2 tsp maple syrup; 2 tbsp olive oil; 2 leeks, halved and finely sliced; 350g chestnut mushrooms, sliced; small handful of thyme sprigs; 200ml dry white wine; 50g unsalted butter; 50g plain flour; 700ml whole milk; 100g extra-mature cheddar, grated; 100g sourdough, blitzed to breadcrumbs