This follows much the same principle as the mac & cheese I put up back in October. The sauce is really almost entirely the same – you just do it with potatoes instead of pasta. Here is an opportunity to get to know celeriac, if you are not familiar with it, in an easy-to-handle format. I really like celeriac, but it is strong – a whole dish of it alone is usually too much for the uninitiated.
Preheat the oven to 375.
2 c. milk (I use skim or lowfat and it works fine either way)
1/4 yellow onion, peeled, stuck with two cloves and a bay leaf
2.5 lbs baking potatoes, peeled and sliced about a 1/4 in. thick
1 celeriac peeled (cut the ends flat, then set it on your cutting board and cut the skin off using a knife. If you try to use a vegetable peeler on it, you’ll just end up with a mess & a high level of frustration) and halved top to bottom, then cut in 1/4 in. slices
Place the milk, onion, potatoes, and celeriac in a deep saucepan and bring to a gentle simmer. Simmer for 10 minutes, then remove from heat. Remove the potatoes with a slotted spoon and place in an 8×8 or slightly larger casserole dish. Season with salt and pepper. Remove the onion from the milk and reserve, but discard the attached seasonings. Chop the onion into long, thin slivers and set aside.
While the milk is simmering, prepare a roux using:
4 T. butter
4 T. flour
and cook over low heat for 5 minutes, stirring constantly. Set aside and allow to cool.
Combine the warm milk and the cool roux in the large saucepan and place over low heat. Cook together until the milk thickens – do not allow to boil! Additionally, remember that the cheese will bind the sauce further, so don’t over-thicken it.
In the meantime grate:
A total of about 9 oz. of sharp & mild cheeses (Last night I used about 2 oz. of Tillamook cheddar, about 5 oz. of cave-aged Gruyere, and about 2 oz. of Dry Jack. My favorite cheddar to use is Grafton 2 Year, but I always use at least two cheeses, often three. Leftover bits of any other kind of cheese are a fine addition.)
10 oz. frozen cut leaf spinach
in the microwave for five minutes – no need to add water. Remove and place in a sieve; press out as much moisture as possible, (a potato masher works great for this) then chop into fine pieces & season with salt and pepper.
When the sauce has thickened, add the grated cheese and stir occasionally until incorporated. Again, do not allow to boil. Taste for seasoning and add:
the slivered onions
salt and pepper if needed
1 tsp. to 1 Tbsp. Moutarde de Meaux or Dijon mustard, depending on how much you like mustard
1 tsp. paprika
a few chili flakes or cayenne pepper (I do about 1/2 t. cayenne)
the cooked, drained spinach
and stir until combined. Pour over the potatoes in the casserole and mix in gently. Bake for 35-40 minutes or until the top is slightly browned and the dish is bubbling. Makes 4 large servings.