Slice the large tomatoes into thick rounds and halve the cherry tomatoes. Arrange them on a large serving platter or in a shallow bowl.
Toss the sliced onion with sumac and scatter over the tomatoes.
Add the avocado, basil, and mint on top. Drizzle generously with olive oil and a splash of red wine vinegar. Season with flaky salt and freshly cracked black pepper. Serve immediately