Slice cucumbers. Cut tomatoes to your liking. I cut in slices and then each slice in half. Slice red onions thinly. Separate rings.
Combine cucumber, tomatoes and red onion pieces in a large bowl.
In a mason jar with a tight fitting lid or a dressing shaker combine the olive oil, canola oil, red wine vinegar, Dijon mustard, salt and pepper. Shake until fully combined.
Pour over vegetables. Serve immediately or refrigerate until serving.