Obviously, this is something we amateurs are hardly capable of. It's not so easy to cook fish properly, and nothing is better than a perfectly cooked fish fillet: neither too much (it dries out and hardens), nor too little (it's not sushi, being "pearly" when cooked isn't always enough).
You might think that it's only a question of time: if you put your tournedos on the stove for x minutes, it will be cooked to perfection, or if you put your roast in the oven for y minutes, it will be too. But no, it doesn't work like that, because the heat source is highly variable: you put your frying pan on the fire, and it takes a certain amount of time to reach the right temperature, depending on your setting; your meat may have just come out of the fridge, so it's cold, which means it will take longer to cook, whereas meat at the right temperature will cook faster, in short... It's almost mission impossible.
So, should we give up? No, in fact, technology can come to our rescue in a very simple way: since cooking means reaching a certain temperature, and this temperature can be measured with a thermometer, we can rest easy enough on that.
Returning to the story of the undercooked fish, it's enough to know that fish is (generally) perfectly cooked if it reaches 60°C at the core. To cook your fish successfully, put it in the oven or on the stove, stick a thermometer probe in it and wait for it to reach the famous 60°C, then remove from the heat and serve within 3 to 5 minutes. You'll be amazed at the results.
It works for just about any type of cooking, and above all, it's completely environmentally friendly: is my oven/oven hot? Is my meat or fish at room temperature or not? Is the thickness of my foie gras even? Is my sugar "grand cassé" rather than blond caramel? None of this matters, only the core temperature of your dish.
If you're going down this road, you'll need a good thermometer: opt for an electronic one that goes up to 300°C, with an oven-safe stainless steel probe (some kitchen ovens now include a probe), and if possible a temperature alarm: it beeps loudly when the indicated temperature is reached, allowing you to do something else while cooking. It's not cheap, but it's a good investment, and once you've tried it, you'll see that it's a life-changing device.
To sum up: when cooking, only the core temperature counts, and the thermometer is ideal for that.
