In a small bowl whisk together 3 tbs extra-virgin olive oil, 1 tsp chopped rosemary, 1 tsp chopped thyme, 1 tsp parsley, 2 minced garlic cloves, 1 tsp Dijon mustard, 1 tsp kosher salt, 1 tsp freshly ground black pepper and 1 tsp lemon zest until combined.

Marinate steak

Pat the filet dry with paper towels. Place the 8 oz filet (or substitute a 6 oz petite filet if desired) in a shallow dish or resealable bag. Pour the marinade over the steak, turning to coat all sides. Cover or seal and refrigerate for at least 30 minutes and up to 4 hours. Remove from fridge 20–30 minutes before cooking to come toward room temperature.

Sear and cook

Preheat a heavy skillet (cast-iron recommended) over medium-high heat until very hot, about 3–4 minutes. Add a thin film of oil or brush the skillet lightly. Pat excess marinade off the steak, leaving herbs on the surface. Sear the steak 2–3 minutes on the first side without moving to develop a brown crust. Flip and sear the second side 2–3 minutes. Reduce heat to medium. Add 1 tbs unsalted butter and a sprig of rosemary or thyme to the pan; tilt the pan and spoon melted butter over the steak (basting) for 1–2 minutes.

Finish to temperature

For rare, cook to an internal temperature of 120–125°F (49–52°C); medium-rare 130–135°F (54–57°C); medium 140–145°F (60–63°C). Use an instant-read thermometer inserted into the center to check doneness. If the steak needs more time, finish in a preheated 375°F (190°C) oven for 3–6 minutes depending on thickness. Remove steak from pan and rest on a cutting board tented loosely with foil for 5–7 minutes to allow juices to redistribute.

Mashed Potatoes

While the steak is marinating (or when you begin cooking the steak), place peeled and cubed potatoes (1 lb) in a medium pot and cover with cold water by 1 inch. Add 1 tsp salt. Bring to a boil over high heat, then reduce to a simmer and cook until potatoes are fork-tender, about 12–15 minutes.

Drain potatoes well and return to the warm pot off the heat for 1 minute to allow excess moisture to steam off. Meanwhile heat 1 cup whole milk and 3 tbs butter together until warm (do not boil). Mash the potatoes with a potato masher or ricer until smooth. Gradually stir in warmed milk and butter until you reach desired creaminess. Stir in 2 tbs sour cream if using. Season with 1 tsp salt and 1/2 tsp freshly ground black pepper to taste. Garnish with 1 tbs chopped chives. Keep warm until serving.

Asparagus

Preheat oven to 425°F (220°C) if roasting, or heat a skillet over medium-high heat if pan-searing. Toss trimmed asparagus (8 spears) with 1 tbs olive oil, 1/2 tsp salt and 1/4 tsp black pepper.

To roast: spread asparagus in a single layer on a rimmed baking sheet and roast 8–10 minutes, until tender-crisp and slightly blistered. To pan-sear: add asparagus to the hot skillet and cook, turning occasionally, 6–8 minutes until tender and lightly charred. Finish with a squeeze of lemon from the wedges and keep warm.

Finishing & Plating

Slice the rested filet against the grain or serve whole. Plate a generous scoop of mashed potatoes, arrange asparagus alongside, and place the filet on top or beside the potatoes. Spoon any reserved pan juices or the butter-herb basting from the skillet over the steak. Garnish with the herb sprig and serve with lemon wedges on the side.