For me, simple is best. Fresh loaf of Italian bread, cut to size roasted red bell, buffalo motz, basil and a drizzle of good olive oil. Maybe some thin slices of Prosciutto di Parma or San Daniele.
If you Google 'Ricette pannini', and 'Ricette pannini gourmet', you will find dozens of illustrations of Italian pannini that show very clearly what the ingredients are. Happy browsing!
It was advertised as a panini, but ended up being only grilled bread, though was really good, roasted turkey breast, brie cheese, and thinly sliced, very lightly sauteed, just enough to soften a bit granny smith apple.
I prepare a few different panini from time to time ..
1) Goat cheese with sun dried tomatoes.
2) Rosemary Lemon Chicken Breast with fresh Spinach Leaves.
3) Mozzarella di Bufala with fresh tomato and fresh Genovese basil ..
4) A " Ruben " which can be made traditionally or with turkey breast ..
5) A Spanish Panini, Manchego Cured Cheese with Sautéed Mushrooms ..