Preheat oven to 400°F. Use a shallow oven safe lidded pot that will hold the fish fillets in a single layer. Alternatively, you can use an oven safe dish and cover it tightly with foil. Melt the butter in the pot then add the olive oil, sliced fennel and onion or leek and stir to coat. Sprinkle with a couple pinches of salt. Add thyme sprigs, if you're feeling ambitious you can strip the leaves from the stems. If it's tomato season, add the cherry tomatoes. Bake for 30 minutes until vegetables are tender.
Place fish fillets atop vegetables. Sprinkle each with a pinch of salt. Spritz lemon over the top of all, pour in the wine, cover, and back into the oven. Cook for 10 minutes then remove from oven and let steam an additional 5 minutes before serving. Garnish with reserved fennel fronds.