I make a fabulicious cole slaw. I bet the dressing would go nicely in tuna. You may need to adjust the quantities a bit, but here's what I do for a full batch of cole slaw:
Grate one apple. (I use a firm, fairly sweet variety like Pink Lady or Gala.)
Grate one yellow onion. (Wash hands.)
Put this, along with most of the juice they generate, into a bowl. (If you really loathe raw onion, nuke it for half a minute to a minute to take the edge off.)
Stir them together. Add a huge blob of mayo. (Start with half a cup. You may need to add up to a cup, depending on how much onion and apple you use.) Add a generous dose of mustard (if you're using a squirt bottle, you'll have to do two full squeezes; if not, two generous teaspoons is a good starting point).
Salt with abandon. Pepper happily. Add 2 T. of white sugar. Stir, stir, stir. Add about 2 T. of cider vinegar. Stir. If the dressing doesn't look at least a bit liquidy, add two more 2 T. vinegar. Stir stir stir. Add to your heap of cabbage.
Obviously, that's not tuna. For the tuna, you might want to mince your apples and onions for added texture. The secret is to mix everything but the tuna together and then add it to the tuna.