In a large pot over medium heat, heat the oil until shimmering. Add the leeks and celery and cook, stirring occasionally, until softened, about 3 minutes. Add the garlic and thyme and cook until aromatic, stirring, 30 seconds more. Stir in the potato, salt and pepper, then add the stock and bring to a boil. Reduce the heat to medium-low and simmer, uncovered, until the potatoes are tender, 10 to 15 minutes.
Using a slotted spoon, remove about 1 cup of the vegetables from the pot and set aside. Add the spinach and parsley to the pot and cook until wilted, about 30 seconds. Remove the pot from the heat and, using an immersion blender, puree until smooth (alternatively, let cool, then puree in several batches in a blender). If not planning to serve the chowder right away, refrigerate or freeze until needed.
Return the pureed soup to a lively simmer, add the fish and scallops and simmer until both are just barely translucent in the center, 3 to 5 minutes. Stir in the reserved vegetables and lemon zest and juice. Taste, and season with additional salt, if needed, then remove from the heat.