Melted leeks, I find, are one of those preparations that taste super indulgent without really being as such. Cooking down a mountain of leeks with some white wine, butter and herbs into a creamy side dish with just a bit of that beautiful spring onion flavor is like heaven in a skillet. Keeping things light, yet satisfying, I added some mushrooms to the mix and topped it all off with a piece of seared halibut spiked with a cap of classic Italian gremolata. Melted leeks are also a great transition-into-warmer-weather side dish: light and springy, yet satisfying enough for a night's sleep with a bit of chill still stuck on it.
Return the skillet to medium heat with 1 tablespoon olive oil and the butter. Add the leeks to the pan, season them with salt and pepper, then cook, stirring frequently (don't let them get brown), until they've softened, about 10 minutes. Add the white wine and dried oregano to the skillet along with the reserved mushrooms and continue cooking, stirring occasionally, until most of the wine has evaporated, about 5 minutes more. Season as needed with salt and pepper; reserve warm.
When the leeks are just about ready, place a second medium skillet over medium-high heat with the olive oil. Season the fish liberally with salt and pepper, and sear it flesh-side down first, turning the fillet only once, until cooked to your liking, about 5 minutes per side for medium (depending on the thickness).
Serve the melted leeks topped with a piece of fish per plate and a garnish of the gremolata.