Oven – When a recipe calls to reduce oven temperature mid-cooking, should you leave the door ajar until the oven reaches said temperature

When you're determining the doneness by time, you're doing it wrong :)

There are too many differences between individual fish to go just by weight, thickness, oven temperature, and time. The best way is to measure the internal temperature (stick a thermometer into the thickest part of the fillet); I like about 120 F (50 C) for salmon (carryover cooking raises the temperature a little higher after taking it out of the pan), but then it has to be good quality fish. You need to check early and often, because it can go from undercooked to overcooked fairly quickly. Now, like zanlok says, my experience is mostly with pan frying, and I'd expect using the oven would be a bit slower than using a pan because of slower heat transfer; so you should be a bit safer.

If you don't have a meat thermometer, an easy way to test fish for doneness is to look for translucency. Salmon can still be just a little translucent in the middle. Also, properly done fish flakes along the natural "seams". Another nice trick (which doesn't work for boneless fillets - sorry) is to find a bone in the thickest part of the fish: if it comes out fairly easily, then the connective tissues have mostly dissolved and it's done.

That leaves the temperature for your oven. I would go with very high - that way the fish gets the least possible time to dry out (which is going to be a big risk with this cooking method, I would think - unless you wrap your salmon in bacon or something like that). At a guess, I'd try 450 F (230 C).

Use a pot of water!

1. Find out the lowest temperature of your oven (needs to be at least below 212 F for it to work). Preheat the oven to this heat
2. Heat water on the stove to a temperature let's say, about 10 F below the oven temp. Use a candy thermometer and leave it in the water, but not touching the bottom of the pan.
3. Once the oven is preheated, transfer the pot of water to the oven, and leave the candy thermometer on the whole time.
4. Periodically check the temperature of the water. It should asymptotically approach the temperature of the oven. If the water temp closes in on the temp of the oven, the oven thermometer is calibrated!
5. You can verify by checking the temperature on the opposite end. Warm the water on the stove to 10 F above the oven temp, and watch the candy thermometer go down to the oven temp.

Note: it is very important to get the water on the stove close to the oven temp. Water has a very large specific heat capacity and will take quite a while to heat up in the oven, as it takes a large amount of energy to heat water.