Crumbling down.

From D., 8 April 2012, celeriac crumble – replicating a restaurant dish?

Q: We had a very nice celeriac crumble as a veg dish in a restaurant but unable to find out how it was made. Any ideas please?

A: Thanks for your question. Celeriac is one of my favorite vegetables – certainly my favorite root vegetable – and a crumble is a great way to show it off.

For me, a crumble should include two components: a fruit or vegetable base, bound together with something light; and a crumbly topping. Fruit crumbles are easy, because fruit’s cell structure is more fragile than that of root vegetables. When you expose fruit to heat, the cells collapse and discharge juice; thickening these with a little cornstarch makes for a fruit-tasty, light binder. Root vegetables are different, though; lacking that same cell structure, they soften and break down on heating, but they don’t provide any juice. So you’ve got to make a binder yourself.

Some people like a starch-thickened dairy binder, like a bechamel. I find those kinds of binders unappealingly heavy and starchy; they also tend to become thick and gloppy on cooling. So I recommend instead that you make a potato- and cream-enriched puree of the celeriac, and use that as the cooking medium for the thinly sliced celeriac. It won’t be as solid, but I think that’s a real virtue. Top that with a crumble, made more like a biscuit (because it doesn’t have sugar to hold it all together), and you’re golden.

Celeriac crumble

When trimming celeriac, slice cleanly through to remove the super-knobby, hairy end. Don’t try to trim around it; it’s a mess and you’ll just waste your time. Cut it all off to leave a flat surface. Place flat side down on your cutting board and then, using a chef’s knife, slice off the exterior from top to bottom, working around the celeriac until it’s clean. Then you can slice or dice it up.

2 medium celeriac, peeled
1 medium (about 2″ diameter) yellow potato unpeeled
4 c chicken stock or, for a vegetarian dish, vegetable broth
3 cloves garlic confit
1 bay leaf
4 branches thyme
1/2 c heavy cream
4 tbsp cold butter
celery salt
piment d’Espelette or cayenne pepper

1 1/2 tsp baking powder
1 1/2 c AP flour
3/4 tsp kosher salt
1 oz Gruyère, finely shredded
5 tbsp butter, cubed, ice cold
2/3 c heavy cream

For the crumble:

Sift together the dry ingredients and cheese Cut together with the butter into coarse, pea-sized bits. Stir in the cream, just to moisten. Cover and keep chilled until use.

Oven 400F/205C.

Slice all the celeriac about 1/8″ thick. Bring 4 c stock to a boil and drop in the celeriac, the whole potato, bay, and thyme. Cover, reduce heat, and simmer until the celeriac and potato are tender. Drain into a clean bowl, reserving the cooking liquid. Discard the herbs. Peel and slice the cooked potato.

[Alternative: If you are inclined toward sous vide cooking: Seal the celeriac in a bag with a large pinch of celery salt, bay, thyme, and 2 tbsp butter. Set immersion circulator to 183F. Drop in the bag and the whole potato and cook for 40 minutes. Remove from the water bath, discard herbs.]

Divide the cooked celeriac in two portions, about 2/3 and 1/3. Transfer the smaller portion to a blender/vitaprep and blend with about 1 c reserved cooking liquid (or 1 c stock if you cooked sous videe), the cooked potato, and the garlic confit. The mixture should be a fairly thick puree. Add the cream and butter and continue to process until smooth. Season with celery salt and espelette pepper to taste. Combine with the remaining celeriac and transfer to a gratin dish. Top with the cold crumble mixture. For an elegant presentation, you can use individual ramekins/gratins instead.

Bake until the crumble topping is golden brown, about 15 minutes. Let sit about 5 minutes or so before serving.