Fix a few hours ahead, if possible.

Tomatoes
Onions
Sal t
Pepper
Dried basil (who not fresh!)
Sugar
Vinegar
Olive oil

Put a layer of sliced, unpeeled tomatoes in a shallow bowl. Put a layer of sliced onions on top. Sprinkle with salt and pepper, a pinch of basil, a tiny bit of sugar (or omit) and one teaspoon each of vinegar and olive oil. Add another layer of tomatoes and onion slices and repeat the seasonings. Continue to layer until you have the amount you need.