2 sweet potatoes, peeled and thinly sliced
1 (1-pound) bag baby carrots, halved
2 cups cauliflower florets, sliced
1 celery root, peeled and sliced
3 tablespoons olive oil, divided
1 teaspoon fresh thyme, chopped
1 teaspoon salt
1 teaspoon pepper
1 medium onion, diced
3 tablespoons tomato paste
2 cups reduced-sodium chicken broth
1/2 cup mild cheddar cheese, shredded
2/3 cup quick oats
1/4 cup ground hazelnuts
2 tablespoons all-purpose flour
2 tablespoons honey
Preheat the oven to 400 degrees F.
Grease a 9×13-inch casserole dish.
Place a large sheet of parchment paper on a baking sheet.
Toss the sweet potatoes, the carrots, the cauliflower, and the celery root with 1 tablespoon of the olive oil, the thyme, the salt, and the pepper.
Transfer the sweet potato mixture to the baking dish and roast until the vegetables are tender and golden, about 25 minutes.
Heat 1 teaspoon of the olive oil in a saucepan over medium heat.
Add the onions to the hot oil and cook until softened, about 3 minutes.
Stir in the tomato paste and cook until fragrant, about 1 minute.
Add the broth and bring the mixture to a boil.
Boil the mixture for 2 minutes.
Transfer the saucepan from the heat and stir in the cheese, continuing to stir until it melts.
Add the roasted vegetables to the cheese sauce, stirring to combine.
Pour the vegetable-cheese mixture into the prepared casserole dish.
Combine the oats, the hazelnuts, the flour, the honey, and the remaining olive oil to form a crumble.
Sprinkle the crumble mixture on top of the vegetables.
Bake until the topping is golden-brown, about 30 minutes.