Melt butter in medium saucepan over medium heat. Add leeks and cook for 2 minutes, stirring frequently. Reduce heat to low, cover, and cook, stirring occasionally until leeks are very tender. (About 10 minutes).
Increase heat back to medium and add lemon juice. Stir until liquid evaporates (about 1 minute) and then mix in cream. Let simmer until slightly reduced. (about 2 minutes).
Remove from heat and cool slightly.
Transfer to blender and blend until smooth. Season with salt and pepper to taste.
Serve on top of broiled salmon, talapia, grouper, halibut, cod --
For simple fish: Preheat broiler. Place fish skin side down in roasting pan. Sprinkle with salt & pepper and dot with butter. Broil (4 inches from heat) 5-6 minutes or until browning. Make a cut in thickest part to ensure it's done and flaky, then use metal spatula to lift off from the skin, serving browned side up.