Potato and Celery Root Gratin With Caper Brown Butter

Step 1

Heat the oven to 375 degrees Fahrenheit/180 degrees Celsius.

Step 2

Add 2 tablespoons/30 grams of butter and all the oil to a medium (10-inch/26-centimeter) ovenproof sauté pan over medium-high heat. Once melted, add the onions and 1/2 teaspoon salt, turn the heat down to medium-low and cook, stirring occasionally, until onions are softened and have taken on a light coloring, about 15 minutes. Add the garlic and thyme leaves, and cook for another 3 minutes, stirring occasionally, until fragrant. Stir in the miso and wine, and cook until liquid is reduced by half, about 4 more minutes. Remove from the heat.

Step 3

In a large bowl, very gently toss together celery root (celeriac), potatoes and flour with 1 1/2 teaspoons salt and a generous grind of pepper. Stir in the onion mixture to coat, then transfer everything back into the skillet, piling the vegetables haphazardly, so that the batons are pointing in different directions. Pour in the stock. Lightly grease a piece of foil with a little oil, then tightly cover the pan (oiled side down) and bake for 1 hour, or until the vegetables have cooked through.

Step 4

Remove the foil, and turn the oven temperature up to 450 degrees Fahrenheit/210 degrees Celsius. Sprinkle over the Gruyère and return the dish to the oven for 30 minutes, or until nicely browned on top, rotating the dish halfway. Leave to settle for at least 20 minutes while you make the brown butter (beurre noisette).

Step 5

Add the remaining butter and the thyme sprigs to a small saucepan, and place it over medium heat. Once melted, cook for 3 to 5 minutes, stirring occasionally, or until it starts to smell nutty (add more or less time if necessary). Add the capers (carefully, as they might spit) and cook for 4 more minutes, or until they darken and burst. Off the heat, stir 1 tablespoon lemon juice from 1 lemon, chives and another good grind of pepper.

Step 6

Pour the caper brown butter all over the top of the gratin and serve warm, with the remaining lemon sliced into wedges to squeeze alongside.