Preheat the oven to 400 degrees F.
Melt the butter in a deep pan, add the leeks, and cook for 2 to 3 minutes. Then add the shallots and sweat for another 3 to 4 minutes. Add the beer, season, and then add the fish stock, reduce.
Place the fish fillet on top of the leek/shallots mixture and place the slices of tomatoes attractively on top. Season the fish and place into the oven for 10 to 12 minutes.
When finished, place the fish on a plate, add a little piece of butter in the sauce to thicken and smooth it, check the seasoning, and pour on the fish. Serve immediately.