In a small bowl, whisk together the mayonnaise, sour cream, mustard, and sweetener until smooth and well combined. Add the lemon juice, salt, pepper, and celery salt, whisking until the dressing is fully emulsified and seasoned evenly. I prefer to make the dressing first so the flavors have time to meld while I prepare the vegetables—it makes a noticeable difference in the final taste.

While the dressing sits, finely dice the red pepper into 1/4-inch pieces and finely dice the onion. Chop the fresh dill and drain the tuna thoroughly in a strainer, pressing gently with a spoon to remove excess water—this prevents the coleslaw from becoming watery. Set all prepared ingredients aside.

Place the coleslaw mix in a large mixing bowl and gently break up any compacted clumps with your hands. Add the diced red pepper, onion, drained tuna, and chopped dill, tossing everything together to distribute evenly. I like to fluff the coleslaw as I add each ingredient to prevent it from matting down—this helps it maintain a light, crisp texture.

Pour the dressing from Step 1 over the coleslaw mixture and toss thoroughly until every piece is coated and the ingredients are evenly distributed. Serve immediately for the best texture, or refrigerate for up to 2 hours if needed—avoid making it ahead by more than a few hours, as the coleslaw will gradually soften and release excess liquid.