In a small mixing bowl, whisk together balsamic vinegar, olive oil, and minced garlic until smooth and emulsified. Set aside.

Slice heirloom tomatoes into wedges, about 1/2 inch to 1 inch wide, and arrange on a serving platter.

Soak thinly sliced red onion in cold water for 10 minutes, then drain and pat dry.

Layer tomato wedges on the serving dish, scatter soaked red onion slices, and crumble feta cheese on top.

Drizzle the vinaigrette over the salad just before serving, using as much or as little as desired.

Gently toss the salad to combine without squashing the tomatoes. Serve immediately.