Is there any general guidelines for preventing a cheese and cream based Gratin from "breaking", i.e. fat separating. The last two I have made have turned into oily messes, even though I seem to cooking it properly: bubbling and beginning to brown.
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Depending on what youi're using for vegetables, the breaking could be caused by too high a temperature in your oven. You might try baking at 350-375 with a foil tent, then browning the top with a broiler for the final few minutes. You could also try baking in a water bath. My final thought would be to make the gratin with a bechamel: Make a roux with 2 Tbsp butter and 2 Tbsp flour for every 2 cups of cream, then simmer the sauce for 15 minutes before pouring into your gratin.
chef arik is right on here. too high a heat and/or too long cooking.
Agreed. You might also have trouble if using veggies that exude a lot of moisture.