Trim both ends of cucumber. Cut in half and slice in thin strips, using either a food processor or mandolin. You can also slice it crosswise, and a knife will work just fine. Do the same with red onion.
Cut the tomatoes in half, and cut the asparagus in 1 inch pieces on the diagonal.
Place all the vegetables in a bowl. In a separate bowl, mix the vinegar, water and sugar. Pour over the vegetables and toss.
Allow to sit for at least 10 minutes to take in the flavor. Arrange on a platter to serve.