Preheat oven to 200C. Prepare vegetables: cut cauliflower into small bite sized florets. Peel and cube potatoes into 2-3cm bite size chunks. Pick thyme leaves from stalks.
Combine all spice mix ingredients. Spread cauliflower and potatoes onto two large roasting trays. Drizzle with olive oil, then coat with spice mix, using your hands to ensure even coverage. Roast for 25-35 minutes, until cooked through and golden on the outside.
In a medium bowl, combine crumb ingredients and mix well.
In a large pot, heat a generous lug of olive oil and vegan butter over medium heat. Add onion and saute for 3-4 minutes until translucent, then add garlic and continue to stir for 2 minutes until fragrant.
Add soy milk, vegetable stock powder, mustard and nutritional yeast and stir until it thickens slightly and forms a sauce, roughly 3 minutes, then turn off heat. Add cannellini beans, roasted cauliflower and potatoes and mix to combine.
Transfer mixture to ovenproof dish. Pat down the gratin so it is level, then spread the crumb mixture evenly over the top. Place in the oven on the grill/broil setting on medium-high for about 7 minutes or until the crumb is golden and crispy (check regularly so it doesn't burn!)