Did 150 °C (300 °F) ruin my cream sauce, or is it just overcooked, or fine? I made a new recipe -that had chicken, mushrooms and onion sauteed in butter, to make a casserole.  I made a white sauce - butter/flour, lactose-free milk, and it thickened very slowly, so added some cornstarch, and it thickened nicely.
As the recipe said, I whisked in one cup of heavy cream, poured it over the chicken/mushroom/onion garlic mixture, then put it in a 13x9 glass tempered dish, covered with foil, tightly, and put it in an oven at 150°C (300°F); and it was in there for two hours, maybe 2.5.  

When I got it out, it was brown, through and through, and it looked like butter had separated; no white cream sauce to be seen.  The flavors were separate, and althoguh I didn't like it, my husband ate it, and I am thinking it was burned through and through -- but I am not sure this was the way it was supposed to turn out. 

The original instructions didn't have 2.5 hours at 150°C (300°F), but 100°C (225°F), for 2-3 hours - and the recipe used a dutch oven, which I don't have. The higher heat was an oversight.

Did I burn it?  Is it just overcooked, and edible, or is it bad?  I have absolutely no idea at this point, but I have been reading that cream sauces have a top heating level of 200, so I think I really blew it with the 300 degrees.
I think it would have been better to have kept the sauce separate and make it after the casserole has cooked.  White sauce needs the temperature reducing as soon as it starts to crack; indeed, lift it off the heat as soon as you feel this taking place. It is possible almost to thicken it completely from the heat already in the pan.