Chill watermelon slices with salt in a re-sealable bag for 2 to 3 days.
Preheat your oven to 150°C.
Toast sourdough, slice widthways, and bake between trays for 5 minutes until crisp.
Flatten tuna steaks between cling film to a 2mm thickness using a mallet.
Trim flattened tuna into circles using an upturned bowl as a guide.
Mix finely chopped tuna trimmings with half the lemon juice, chives, shallots, and seasoning.
Fry chopped watermelon in browned butter for 1 to 2 minutes per side until caramelised.
Spoon tartare onto toast with watermelon and serve alongside tuna carpaccio brushed with oil and lemon.