Cube your heirloom tomatoes to about 1 1/2 inch thick cubes. Place in a large bowl and set aside. This recipe works well with meaty tomatoes like beefsteak or green zebra.
In a medium bowl, combine the anchovies, lemon juice, lemon zest, and white balsamic, and whisk together until well combined and the anchovies start to dissolve. Add the thinly sliced onions and let them sit for about 15 minutes in the mixture to mellow their flavors, making sure the onions are coated in the liquid. If you need to toss them every few minutes, this works too.
Once the onions have mellowed slightly, drain out the liquid into a medium size bowl (you will use this to make the dressing) then add the onions to the tomatoes.
To the lemon juice mixture, add the olive oil and whisk to create an emulsion. If it doesn't thicken, add a little more olive oil until you get a consistency you desire.
Lightly toss the heirloom tomatoes and onions with the anchovy dressing. Top with fresh oregano leaves and fetta cheese. Salt and black pepper to taste. This recipe is also nice with chili flakes.