Ingredients:
• 1 pound lamb, sliced thinly across the grain
• 3 tablespoons butter (divided: 1 tablespoon for searing, 2 tablespoons for sauce)
• 1 tablespoon olive oil (or neutral high-smoke-point oil)
• 8 oz. mushrooms, sliced thick
• 1 medium yellow onion, finely diced
• 1 tablespoon tomato paste
• 1 to 2 tablespoons Dijon mustard (start with 1, add more to taste)
• 1 cup warm chicken or beef stock
• 1/2 cup sour cream
• 2 teaspoons fresh lemon juice
• salt
• black pepper
• parsley, finely chopped
• lemon zest, finely grated
• cooked pasta, hot
Directions:
• Pat the lamb slices thoroughly dry with paper towels and season lightly with salt and pepper.
• Heat 1 tablespoon of olive oil and 1 tablespoon of butter in a large, wide skillet over high heat until sizzling hot. Add the lamb in a single layer (work in two batches if your pan is small).
• Sear for 60 to 90 seconds total, turning once, just until browned on the exterior while remaining tender inside. Immediately transfer the lamb and any resting juices onto a plate and set aside.
• Return the skillet to medium-high heat.
• Add the sliced mushrooms to the dry pan residue.
• Let them brown undisturbed for 2 to 3 minutes, then toss and cook for another 2 minutes until deep golden.
• Transfer them to the plate with the lamb.
• Lower the heat to medium and melt the remaining 2 tablespoons of butter in the skillet.
• Add the diced onion with a pinch of salt.
• Cook for 4 to 5 minutes, stirring occasionally, until soft, translucent, and lightly golden.
• Stir in 1 tablespoon of tomato paste.
• Cook, stirring constantly, for 1 to 2 minutes until it darkens to a brick-red color.
• Whisk in the Dijon mustard and pour in the 1 cup of warm stock, scraping up all the browned bits stuck to the bottom of the skillet.
• Bring the liquid to a gentle simmer and let it reduce by about one-third, roughly 4 to 5 minutes, until it coats the back of a spoon.
• Turn the heat down to low.
• Whisk in the room-temperature sour cream and lemon juice until the sauce is completely smooth and velvety.
• Taste and adjust with salt and plenty of cracked black pepper.
• Return the seared lamb, browned mushrooms, and all accumulated resting juices back into the skillet.
• Let everything warm through on gentle, low heat for 1 minute—do not let the sauce boil, or the sour cream may split and the lamb will toughen.
• Toss the hot pasta directly into the skillet with a splash of starchy pasta cooking water if you prefer a silkier glaze, or ladle the sauce generously over individual nests of pasta.
• Finish with chopped fresh parsley and a dusting of lemon zest.
