Making a tosti seems straightforward enough. However, a good tosti needs some attention. Tomatoes are forbidden. There is too much water in tomatoes. It'll make the tosti mushy.
Get the best bread possible. I used sliced Grand-mère (white) baked by Vlaamsch Broodhuys. It's a traditional sourdough bread.
Assemble the tosti: slice of bread, a slice of a good quality young farmer's cheese, you need the rubbery texture when the cheese melts, some slices of good quality ham (not glued together in a factory), red onion, thinly sliced, another slice of cheese, slice of bread.
Butter BOTH sides of the tosti and fry in a pan.
Flip over frequently until both sides are golden brown.