From a blog called: eatlifewhole.com
2 cups Roasted Cauliflower
2 cups butternut squash, peeled & chopped
3 tablespoons olive oil
1 medium yellow or sweet onion, sliced lengthwise
1 1/2 cups Gruyere cheese, shredded
1/3 cup walnuts, toasted & chopped
1 tablespoon fresh rosemary, chopped
Sea salt & freshly ground pepper
Preheat the oven to 425 degrees. Prepare the cauliflower.
While the cauliflower is in the oven, prepare the squash for roasting. Slice off both ends and peel completely. Cut the squash in half and scoop out the seeds with a spoon. Continue cutting into bite-size pieces.
Add the cut squash to a bowl. Coat with 1 tablespoon olive oil. Generously sprinkle with salt & pepper. Move to a parchment-lined baking sheet. Roast the squash until it is cooked through but not mushy, about 30 minutes.
While the cauliflower and squash are cooking, caramelize the onions. Heat 1 tablespoon olive oil over low heat in a medium-sized sauté pan. Add the onions and stir until they are well coated with oil. Cover, stirring occasionally, until the onions are soft, about 25-30 minutes.
When done roasting, remove the cauliflower and squash from the oven. Set aside to cool.
Coat a medium cast-iron skillet or casserole dish with the remaining 1 tablespoon of olive oil. Cover the bottom with 1/2 of the squash and 1/2 of the cauliflower, followed by the onions. Add the remaining cauliflower and squash. Top with the cheese, followed by the walnuts and the rosemary.
Reduce the oven temperature to 400 degrees. Return the gratin to the oven to cook until the cheese has melted and started to brown, about 15-20 minutes.