Preheat your oven to 400 degrees f.
Place the potatoes on a baking tray, prick the potatoes several times with a fork, rub each with a little oil over each of the skins, and season with sea salt. Bake for 40-50 minutes, or until tender through the center.
In a medium bowl combine your drained tuna with the Dijon mustard, 2 teaspoons lemon juice, fresh chives, sea salt, and pepper to taste. Cover, then set in the fridge until needed.
While your potatoes are baking in the oven, make your salad. In a large bowl combine your fresh spinach, halved cherry tomatoes, remaining fresh lemon juice, 1 Tablespoon of olive oil, and sea salt, & pepper to taste. Toss well.
When your potatoes are finished baking, slice through the center of each potato as shown in the photos. Spoon in your tuna mixture to the center and top with Greek yogurt as shown.
Serve potatoes while hot alongside your spinach salad and enjoy.