Well for fish I would do this.
Saute some onion or shallot until soft and not caramelized in butter over a medium heat and you could clarify the butter for the best alternative. When the onion is soft add sliced garlic and cook for another minutes or 2, then add the sliced parsnips along with some cream and whole milk and generally a 2:1 ratio of cream to milk will prevent curdling and just bring to a gently boil. I would then add some fennel fronds and some lemon zest, no juice. In a blender add all of those ingredients and blend and if it's too thick add some liquid, water works. Also try and find a 35% cream where there is no stabilizers and with the only ingredient being cream, it cooks better, much better and actually I avoid any that aren't just cream. And of course always season as you go.
If you can't find a recipe that you like I can assemble one based on what I mentioned above, or change it based on a different protein, all you have to do is ask.