Slice tomatoes to 1/2 inch thickness, and place in a single layer on a large plate with raised edges (to hold in the marinade). Dice the cucumber and sweet onion. Combine the olive oil, rice vinegar, sugar, and salt, and stir to fully combine. Put the cucumber and onion in the marinade and stir to coat. Optional: set aside for a few minutes to marinate. Spoon marinade, cucumber, and onions onto the tomatoes and serve.
This recipe is very adaptable. Your can adjust the amount of cucumbers vs. onions, the size you cut them, and the amount of sugar and salt to suit your tastes.