I brought three heads of fennel and a large celeriac the other week and wanted to cook them together. Looking around the internet the recipes I could find for a gratin suggested making a califlower cheese style floury sauce before baking. I didn't want to make a cheesy and stodgy sauce so came up with my own lighter and simpler recipe.
I chopped peeled and chopped the celeriac, chopped the fennel and par boiled them together for five minutes. Once drained I put them in a ovenproof dish. I poured a small pot of whipping cream over. This is a trick I learned from the Ottolenghi cookbook. They use unwhipped whipping cream in their bakes as it thickens while cooking and is lighter than single or double cream. I also put a small grating of cheddar amongst the dish.
I found a stale roll which I blended into breadcrumbs and added a load of parmesan to give it a zingy cheese kick. The breadcrumbs were spread on top and the dish went in the oven for forty minutes until the fennel and celeraic were cooked and the topping had browned.
It came out pretty good and I served it with steamed kale.
My plan had succeeded and I've come up with a decent recipe for a celeraic and fennel gratin. As they are clean tasting the cream and cheese didn't feel heavy. If I make it again I will do individual portions in smaller oven dishes as I found it's best to have a bit of the topping in each mouthful with the veg.