Simple Tomato Salad with Red Onion and Basil

It's tomato season, so I bought a pound of heirloom tomatoes (these ones cost me $2.00 total), and made this simple, quick salad with them.  I cut them into pretty slices, in an arrangement of shapes, and tossed them with some minced red onion that I soaked in water for 10 minutes to make them milder, a glug of olive oil, basil (hand torn rather than cut so that its edges don't darken), and a pinch of Maldon salt and pepper.

Here is the remainder of that salad, a day later.  I added some cut up pieces of fresh mozzarella (that I patted dry with a towel), and 1/2 Tbsp champagne vinegar.  I actually liked this champagne vinegar version of the salad better then yesterday's since these tomatoes are very sweet but not very acidic.  The remaining liquid is the juices from the tomato and the seasoning that I added yesterday.