This marinated-tomato dish can be prepared several hours ahead of time and served chilled or at room temperature. Peel and slice the cucumbers just before serving.
Slice tomatoes lengthwise into quarters, through the stem. Place in a medium bowl; season with salt and pepper.
Heat oil in a small saucepan over medium heat. Add garlic; cook, stirring, until soft but not browned, about 1 minute. Add vinegar, lemon juice, and coriander. Let simmer 1 minute.
Immediately pour hot vinaigrette over tomatoes, and let marinate at room temperature until completely cooled. Alternatively, cover with plastic wrap and refrigerate until ready to serve.
Just before serving, peel cucumbers, leaving on stripes of skin for decoration, if desired. Halve cucumbers lengthwise; scrape out seeds with a spoon and discard. Cut cucumber into 1/2-inch semicircles. Garnish tomatoes with oregano, and serve cucumbers in a separate bowl.