Scrub small potatoes clean under running water, leaving the skins on — the thin skin adds earthy flavor and extra fiber. Place whole potatoes in a pot of cold salted water, bring to a boil, then simmer for 12-15 minutes until fork-tender (a fork slides through easily with no resistance). Drain and set aside to cool completely
While potatoes cook, hard-boil the egg: place in cold water, bring to a boil, then cover and turn off heat. Let sit for 8 minutes, then transfer to ice water to stop cooking. Peel and cut into wedges once cool
Prep the remaining ingredients: finely dice red onion and chop fresh parsley. Drain canned corn and tuna thoroughly — excess liquid will make the salad watery.
Once potatoes are cool enough to handle, use the back of a spoon to gently crush them into rough, bite-sized chunks. Do not mash into a smooth puree; chunky texture gives the salad better mouthfeel.
Make the dressing first: in a small bowl, whisk together Dijon mustard, whole grain mustard seeds, mayonnaise, salt, and black pepper until completely smooth. Mixing the dressing separately ensures even seasoning instead of clumps of mustard scattered throughout.
Add crushed potatoes, flaked tuna, sweet corn, red onion, and most of the parsley to a large salad bowl. Pour the dressing over top and toss gently to coat all ingredients evenly. Top with egg wedges and remaining parsley, then serve.