2 tablespoons water, plus more as needed
2 tablespoons whole-grain mustard
2 tablespoons Dijon mustard
½ pound (2 sticks) cold butter, cubed
1 lemon, juiced
Coarse kosher salt, to taste
2 tablespoons canola oil
1 head cauliflower, cut lengthwise into 1-inch “steaks”
Capers, to garnish
Parsley leaves, to garnish
Celery heart leaves, to garnish
Toasted breadcrumbs, to garnish
Radish, thinly sliced, to garnish
Sea salt and freshly ground pepper, to taste
Preheat oven to 350 degrees.
Combine water and mustards in a small heavy-bottomed saucepan and bring to a simmer. Reduce heat to medium and quickly whisk in the cubes of butter, one or two at a time, until creamy and emulsified. Remove from heat and season with lemon juice and salt. If you have an immersion blender, you can blend the sauce to help stabilize the emulsion. Have some warm water on hand to lighten up the mixture, as it tends to thicken. Keep sauce warm while you prepare the cauliflower.
Turn head of cauliflower upside down. Cut off the rounded edges to the right and left of the stem and reserve for another use (I like to make cauliflower “rice” with these scraps). You are now left with the center of the cauliflower. Cut lengthwise into even “steaks” about 1-inch thick. A large head should yield 2 or 3 pieces.
Coat the bottom of a 9-inch iron skillet or nonstick sauté pan with canola oil, season with 1 teaspoon salt, and set over medium heat. Add cauliflower to the pan one piece at a time, and cook until sizzling and golden brown. Turn onto a sheet pan lined with aluminum foil and repeat until all the steaks are browned. Roast in a preheated oven for 10-15 minutes, just until tender enough to pierce with a fork.
Arrange cauliflower seared-side-up on a warm platter and drizzle with the warm mustard butter. Garnish with capers, parsley, celery leaves, toasted breadcrumbs and thinly sliced radish. Season with coarse sea salt and a few turns of freshly cracked black pepper.