Creamy Cauliflower, Mushroom & Kale Pasta Bake {vegan option}

Name: Creamy Cauliflower, Mushroom & Kale Pasta Bake {vegan option}
Prep time: 30 min
Cook time: 30 min
Total time: 1 h
Ingredients: 1 small head cauliflower (, , outer leaves removed, cut into medium florets); 1 small potato (, (~80g), sliced 3mm thick); salt; 250 g (9 ounces) wholemeal penne pasta, (I used Barilla); 3 tbsp olive oil; 2 tbsp plain white flour; 200 ml (a scant cup) semi-skimmed milk or unsweetened non-dairy milk, (see notes); 200 ml (a scant cup) vegetable stock; 1 tbsp Dijon mustard; black pepper; 250 g (9 ounces) chestnut mushrooms, , sliced; 5 cloves garlic (, , crushed or finely chopped); 6 sprigs fresh thyme (, , leaves picked & stems discarded); 100 g (3.5 ounces) curly kale, , chopped into bite-sized pieces; 4 tbsp breadcrumbs (, (see notes)); 30 g Parmesan cheese (, grated (optional)); 4 sprigs fresh thyme (, leaves picked & stems discarded); a drizzle of olive oil