This is a slightly modified version of Melissa Clark's recipe featured in New York Times Food
4 cod or hake fillets (6 to 8 ounces each)
Salt and freshly ground black pepper, to taste
2 tablespoons unsalted butter
1 tablespoon extra-virgin olive oil, more as needed
1 large shallot or ¼ of a red onion, thinly sliced
5 sprigs thyme
¼ cup dry white wine
⅓ cup plain yogurt
2 tablespoons minced fresh dill, parsley or cilantro, more for serving
1 small clove garlic, finely grated or minced
Flaky sea salt, for serving (optional)
Cut cod into pieces to fit in the pan. In my case the cod was very thick so I cut it in half horizontally.
Season fish with salt and pepper and set aside.
In a large skillet with a lid, melt butter in olive oil over medium heat.
Add shallot, thyme and a pinch of salt and pepper and cook until golden, about 3 minutes.
Add wine and cook until slightly thickened and shallot is very soft, about 2 minutes longer.
Add cod to pan, spoon some of the shallots over top of fish and cover pan.
Reduce heat as needed; the liquid should be at a simmer, not a boil.
Cook until fillets are just opaque, 2 to 4 minutes.
Meanwhile, in a small bowl mix together yogurt, dill, garlic, and salt and pepper to taste.
Remove the fish with a slotted spoon to a serving platter.
Sprinkle with flaky salt and more pepper, then spoon the shallots and sauce over it.
Drop dill-yoghurt sauce on each piece of fish and sprinkle more fresh dill over that.
Serve the cod immediately. (I served it with mashed potato.)