Preheat your oven to 425 degrees F.
Prep your cauliflower by cutting it into small, bite-sized florets.
Bring a large cast-iron or oven safe skillet to medium-low heat.
Melt the butter in the pan, then add in the sliced white onion and sauté until onions are soft, about 5 minutes.
Then, add in the olive oil and all-purpose flour (use gluten-free AP flour if desired), and stir until everything is evenly mixed. Sauté for another 2 minutes, or until the flour and oil form a paste.
Then, add in the milk and stir. Once the milk is bubbling, add in the pumpkin purée, salt, and spices, and stir until evenly mixed.
Then, bring the heat to its lowest setting. Slowly add in the gruyère cheese one small handful at a time, stirring constantly. Be careful not to let the sauce get too hot - the sauce might separate.
Once all the cheese is melted and there is a smooth sauce in the skillet, add in the cauliflower florets and mix until everything is evenly coated.
Bake the pumpkin gratin in the oven for 25-30 minutes. If desired after baking, broil the gratin for 2-3 minutes for extra browning.
Serve the pumpkin gratin with fried sage, if desired.
Enjoy!