One of the principal ways in which the experience of the home cook differs from the one promised in recipes is that we constantly have to check and adjust our oven temperatures. Even when we're confident about our ovens and utensils, we cannot be sure that a dish put in an oven at gas mark 4/180C for an hour, say, will behave as it is supposed to. This is what I found when I cooked a shoulder of lamb yesterday.
I had studded the joint with garlic and rosemary, anointed it with oil, seasoned it, and left it overnight in a roasting tin in the fridge, with a foil covering. At 8.30 in the morning, I placed it in a gas mark S/130C oven, still with its foil - the theory being that the foil would keep the temperature of the tin below the oven temperature, and so help to cook the meat gently. After a few hours, all was going well.
I peeled and sliced potatoes, and put them into cold water, giving them a good rinse. I then changed the water. I did this because I had decided not to parboil the potatoes, and because I wanted to get rid of as much of their surface starch as possible. Surface starch can cause them to stick, and also forms a tough, rather than crunchy, crust.
At 11.30am I removed the roasting tin from the oven, lifted the joint on to a board, and tipped in the drained potatoes, mixing them with a ladleful of chicken stock and some splashes of olive oil. I seasoned them with salt. I put the joint back on top, covered the tin again, and returned it to the oven. I turned up the temperature to gas mark 3/160C. I wanted the potatoes to steam, before I uncovered the dish and browned them.
After 45 minutes, there was very little activity inside the foil. Eventually, to get things going, I turned the dial right up to 7/220C. I took out the joint at 1.05pm, lifted the tin to a higher shelf, and gave the potatoes 20 minutes to brown, uncovered, while the meat rested.
I have noticed before that a roasting tin, particularly a crowded one, does not heat up very efficiently when covered. Still, the lamb, being a forgiving cut, showed no ill effects from the blasting heat.