Meanwhile, preheat oven to 375°F / 190°C degrees and grease a large tart form with butter (I use a Ø 30 cm PYREX glass form).
Moderately low heat is around 2– 4 on the stove knob. Every stove operates differently, for mine I use setting 3, then setting 4 for the last 10-15 minutes. I use a large non-stick skillet which heats differently than a heavy bottomed one. A heavy-bottomed skillet retains more heat, cooks very evenly and is quicker in my experience. The main thing is, is to slow cook the onion mixture, without browning or burning them.