This veggie-packed sauce will add color and flavor to your dinner plate. Also great with the sauce: salmon or chicken breasts.
Bring a medium pot of water to a boil. Prepare a large bowl of ice water. Salt the boiling water. Add the dark-green leeks; boil for 1 minute. Drain the leeks; transfer to the ice water. Drain the leeks again.
In a large skillet, heat 2 tbsp. oil over high. Season the steaks with salt and pepper. Add the steaks to the skillet; cook for about 4 minutes per side for medium-rare. Transfer to a cutting board; let rest for 5 minutes. Thinly slice the steaks against the grain.
Wipe the skillet clean. Heat 2 tbsp. oil over medium-high. Add the white and light-green leeks; cook, stirring often, until crisp-tender, about 5 minutes. Add the wine; simmer until reduced by half, about 1 minute. Add the cream; simmer until thickened, about 5 minutes. Add 1 tbsp. parsley and 1 tbsp. butter. Season the creamed leeks.
In a food processor, pulse the dark-green leeks, 1/3 cup oil, 1/3 cup parsley, and garlic until smooth. Season the pesto.
Top the potatoes with the remaining 2 tbsp. butter, then the creamed leeks and 1 tbsp. parsley. Spoon the pesto over the steak.