Make the vinaigrette: whisk together the olive oil, vinegar, salt and pepper and set aside.
Boil the new potatoes until tender when pierced with a paring knife (about 10 minutes), removing each potato from the boiling water to a plate. Keep potatoes warm. Save the simmering water to cook the corn. If using fresh corn, quickly blanch in the potato cooking water, and scoop out with a mesh strainer, reserving the cooking water for the spinach. Mix the corn and chives with the vinaigrette.
In a pan large enough to fit the fish without overlapping, combine the stock and wine. Bring to a boil and reduce the heat so the liquid is barely simmering. Carefully slip the fish into the simmering liquid and poach until the fish is just cooked—about 6-9 minutes, depending on the thickness of the fish. Using a slotted metal spatula, carefully transfer the fish to a platter or plates. Cover loosely to keep warm.
Drop the spinach into the poaching liquid and bring the water to a boil for a minute, just until the spinach is bright green. Drain in a colander and press the spinach with a spatula.
Serve fish with potatoes and spinach, top the filets with corn vinaigrette.
443 calories, 18 g. fat, 102 mg. cholesterol, 510 mg. sodium, 32 g. carbohydrate, 6 g. fiber, 40 g. protein