Salt the tomatoes: cut them into wedges or thick slices, sprinkle with salt, and leave for 10 to 15 minutes to release their juices.
Make the pesto: blitz the basil, nuts, garlic, parmesan, olive oil, lemon juice, and a pinch of salt into a loose paste, adding more oil to loosen.
Soak the sliced red onion in cold water for 10 minutes, then drain, to mellow its bite.
Arrange the tomatoes and their juices on a platter and drizzle with the olive oil and a splash of vinegar or lemon.
Tear the room-temperature burrata over the top, spoon over the pesto, scatter the onion and torn basil, and finish with flaky salt and pepper. Serve right away with bread.
Notes
Use the ripest in-season tomatoes you can find. Never refrigerate fresh tomatoes. Add cucumber and olives for a Greek twist, or mozzarella to make it a meal. Best eaten the same day.