Preheat oven to 400°F (200°C). Meanwhile, toss sweet potatoes, carrots, cauliflower and celery root with 1 tbsp (15 mL) olive oil, thyme, salt and pepper. Place on a large parchment paper-lined baking sheet. Roast until vegetables are tender and golden, about 25 min.
Heat 1 tsp (5 mL) oil in a saucepan over medium heat. Add onions and cook until softened, about 3 min. Stir in tomato paste and cook, stirring for 1 min. Add broth and bring to a boil. Boil 2 min. Remove from heat and whisk in cheese until it melts. Add vegetables to cheese sauce and transfer to a greased casserole dish.
Combine oats, ground hazelnuts, flour, honey and remaining oil to form a crumble. Sprinkle crumble mixture on vegetables and bake until topping is golden brown, about 30 min.