Break the rusks into bite-sized pieces. Place the onion slices in a bowl. Drizzle with the vinegar, lemon juice and olive oil. Add the garlic and season with salt. Combine everything very well (keep stirring until the onion wilts). Slice the cucumber in half, lengthwise and deseed it. Dice it in cubes. Put it in a salad bowl. Dice the tomatoes in cubes and add them in, then, add the rusks, olives, mint and marinated onions along with their juices. Give everything a good toss. Chill the salad in the fridge for 15 minutes.