Seared Flank Steak with Fresh Horseradish Cream

From the garden

• 4 tbsp fresh horseradish root, peeled and finely grated, packed · about a 4-inch piece of root; grate just before mixing
• 3 tbsp fresh chives, snipped
• 5 cups arugula, washed and dried

From the pantry

• 1½ lb flank steak, patted very dry
• ¾ cup sour cream
• 1 lemon, juiced
• 1 tsp Dijon mustard
• 1 tsp white vinegar
• 2 tbsp olive oil · divided
• salt · to taste
• black pepper · to taste

1. Make the sauce first so it has time to bloom. Peel the horseradish root and grate it on the fine holes of a box grater or in a food processor; the fumes are strong, so keep your face back and work near an open window. Immediately stir the grated root with the vinegar (this fixes its heat), then fold in the sour cream, mustard, half the chives, 1 tablespoon of lemon juice, and a pinch of salt. Set aside; it gets hotter over the next 10 minutes.
2. Season the flank steak generously on both sides with salt and pepper and let it sit while a large cast-iron or heavy skillet heats over high until it just starts to smoke.
3. Add 1 tablespoon of oil to the pan and lay in the steak. Sear without moving it 4–5 minutes, until deeply browned, then flip and cook 3–4 minutes more for medium-rare (130°F at the thickest part), or a minute longer per side for medium. Flank goes tough past medium, so err on the early side.
4. Move the steak to a board and rest it 8–10 minutes; do not skip this or the juices run out. Meanwhile toss the arugula with the remaining olive oil, the rest of the lemon juice, and a pinch of salt, and spread it over a platter.
5. Slice the steak thinly across the grain, holding the knife at a slight angle. Lay the slices over the arugula, pour any board juices on top, scatter with the remaining chives, and serve with the horseradish cream in a bowl alongside so everyone takes as much fire as they like.