Can I make this a little lighter?
Good question, and the answer is yes. I think this would work well with coconut milk which you can normally use 1:1 swapping for double cream. Of course it will then have a coconutty taste to it, but if you like coconut I think the flavours would still work well. Another option would be to thicken up some regular milk with a slurry of cornstarch - simply mix together some cornstarch with a little water to make a paste and then whisk this into your warm milk. You gratin might be a little more liquidy but I think this would still work. Creme fraiche is another option but I don’t know if that’s any lighter than double cream to be honest. You could add a little flour to the softened onions, to create a roux. Then add some stock and stir well and then stir in a little creme fraiche. Or simply serve a smaller portion of this and have it with green veg and a nice salad on the side.

I have a different type of squash
Perfect! Any squash will do or pumpkin. This would be a great way to use left over pumpkins. I think sweet potato would also work well too. For the sweet potato I might be tempted to boil the slices for 4 minutes before layering them in the gratin.

Beans
I love the hearty richness and the size of butterbeans for this but I think cannellini beans would also work well.

Mushrooms
I think some sliced mushrooms would also be lovely - fry them up until golden, remove from the pan and then sweat the onions. Add the mushrooms back in when you add the beans. I would use tarragon instead of sage if using mushrooms as that’s such a gorgeous combo.

Cheese
I love gruyere so I’ve used a combination of it alongside good old reliable cheddar. Parmesan would also be lovely as would a few slices of mozzarella on the top.

Crumble topping
Someone on Instagram actually suggested this and I thought it sounds great, adding some chopped almonds and breadcrumbs

to the top mixed in with the cheese for a little crunch. Delicious!