What can be done ahead? The dough and onions? Can they be stored in fridge until I'm ready to move on?

Much appreciated. Have a good day!

Much appreciated. Have a good day!

Yes, totally! Well-covered, they'll keep separately for a day or two. (Just don't mix the eggs into the onion until you're ready to bake.)

Thanks so much. Can I also put the dough and onion mixture in the fridge and take it out when I am ready to move on ?

You could definitely complete parts of this recipe ahead! The dough can be prepared through stretching it into the springform pan, the onions can be caramelized and set aside, and the cheese can be shredded. Then you'd just mix up the filling, pour it into the shell, and bake.

But! Tarts are also a fantastic candidate for preparing entirely ahead of time. Just cool and refrigerate, and then serve either at room temperature or warmed in a very low oven to keep it from drying out.