pre-cooked beets, patted dry and cut into ½-inch wedges
Make the dressing: Whisk together the vinegar and mustard in a medium bowl. Slowly add 2 tablespoons oil while continuing to whisk. Add 2 teaspoons water. Season with salt and whisk to combine.
Add beets to the bowl with mustard vinaigrette and toss to coat.
In a separate bowl, combine the spinach with the tomatoes and flake the trout on top. Add the beets and drizzle any leftover dressing over the salad; toss to combine.