Thinly slice the red onion and place in cold water for 10 minutes. Drain and set aside.

Slice heirloom tomatoes into 1/2-inch thick slices and arrange on a platter.

In a large bowl, combine tomatoes, drained onions, and chopped herbs. Drizzle with olive oil and vinegar, then sprinkle salt and pepper.

Let the salad rest at room temperature for 10-30 minutes to meld flavors.

Taste and adjust seasoning with salt, vinegar, or olive oil as needed before serving.