1) "Flat" cooking
Preheat the oven to 210°F (100°C), preferably without fan - quite unnecessary at such a low temperature.
Stick a thermometer
into the thickest part of the liver and put in the oven.
If you'd like to keep the liver "pink", cook until the temperature reaches 122°F (50°C). For "mi-cuit" (just properly cooked, but not overdone), cook until it reaches 149°F (65°C).
If you don't have a thermometer, count 30 minutes cooking for "pink" and 45/50 minutes for "mi-cuit". Unfortunately, these times can only be approximate as your oven and the room temperature will have a marked effect on the cooking. In other words, as soon as you can, buy yourself a thermometer.