- 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!