Prepare the savory custard filling. Combine egg, liquid cream, thyme, salt and black pepper in a bowl to a smooth consistency. Keep aside.
Place rolled out pastry dough into a tart pan and cut off extra on the edges. Poke into pastry with a fork to create small holes. That's so that the pastry doesn't deform when baking blind.
Preheat oven to 400 Fahrenheit/ 200 Celsius.
Place a cut out round baking sheet, which is fitting into the tart base, and keep beans or pie weights on the round cut out baking sheet. Bake your crust alone (called baking blind) for 15 minutes at 400 Fahrenheit/ 200 Celsius. *see notes if you want to skip this step.
Heat up a pan with the butter and sautee onions soft. This can take 10-15 minutes (time added to total cooking time in recipe)
Place cooked onions into the tart crust and spread out evenly. Pour savory custard mixture over the onions.
Bake tart for 30 minutes at 400-430 Fahrenheit/ 200-220 Fahrenheit. *see Notes
Use either 1 rolled out shortcrust pastry (traditional version) or a puff pastry if you want it to be flakier or a filo pastry works great too.
You can skip the blind baking of your pastry crust if you are lazy or if you want to save time and effort. Add the 15 minutes baking to the filled tart baking time. In that case, the total time will be 45 minutes (15 mins just the crust + 30 for the filled tart). BUT keep in mind baking the crust blind has its advantages. The pastry will be better baked through and won't deform.
Adjust baking temperature to your oven. Not all ovens are the same. Start with higher heat and reduce and adjust if need be.