I had exactly the same problem as the OP for years. I read the solution in the Guardian’s (English newspaper) cookery section. Make the omelette as you have described but do not flip. Instead put pan under the grill until topside is cooked as well. If the grill is pre-heated it should only take about five minutes. Slide out of pan, fold if desired. Works a treat for me. Veggie egg scramble is nice, but I really get a kick out of producing a proper Spanish omelette.