Anne,
I think even "when" would be understood that way in English, too, but
it would be much, much rarer than "how" in the same context. And
for heaven's sake, be careful where you put your letters!
Jim
P.S. The French admit to using microwaves??? What will this
do to the reputation of your famous cuisine??? (Or is it only
used to burn trash - like those letters?!!)