Put the onion along one third of the dough.
Top it with tomato.
Then sprinkle on the cheese.
Fold over the dough, sealing the edges well.
I fit three in these pans and "painted" them with raw egg.
Bake in a medium oven until puffed and brown.
The onions, tomatoes and cheese will bake up really well as the dough bakes. There's no need to saute the vegetables first if sliced thin enough. There's more than enough fat in the dough, which is a reason the vegetables cook so well inside.
Use different vegetables
Don't use cheese
Fill with leftovers, but don't tell anybody!
Fill with cooked meat or fish.
Use differnt cheeses.
Please let me know in the comments how it comes out and what suggestions you have, thanks.