Preheat oven to 375 degrees F. Pierce the sweet potatoes using a fork all over and then lay on a sheet of aluminum foil. Drizzle with olive oil to coat evenly and then wrap tightly with the aluminum foil. Place on a rimmed baking sheet lined with aluminum foil and repeat with the remaining sweet potatoes. Bake in the oven for 45-60 minutes, or until tender.
You can gently squeeze the sweet potatoes to test if they’re tender. Carefully unwrap the sweet potatoes from the foil and slice in half with a sharp knife and then let them cool for about 5 to 10 minutes.
While the sweet potatoes are cooling, heat olive oil over medium-high heat in a skillet and add the onion. Cook until tender, then add the baby spinach and garlic and toss for 2 to 3 minutes, until the spinach has cooked down. Set aside.
To cook the chicken, season both sides of the chicken breast with black pepper and garlic powder, and place into olive oil heating over medium-high heat in the same skillet.
Cook, flipping occasionally, until both sides are golden, and the chicken is cooked through, about 4 to 6 minutes per side. Remove from the pan and dice into bite-size cubes, then set aside.
Once the sweet potatoes are cool enough to handle, carefully scrape the sweet potato out of the skin, leaving a thin layer inside with the skin so that it can still stand up on its own.
Add the sweet potato to a large mixing bowl, along with the Greek yogurt, chicken, and the spinach mixture. Stir to combine and season with any additional pepper, as needed.
Coat the potato skins with a drizzle of oil and place back into the oven for 10 to 15 minutes to get a nice crispy skin.
Remove from the oven and fill each skin with the sweet potato mixture, topping with shredded Swiss cheese. Bake again for 10 to 15 minutes, or until the cheese has melted and the filling has heated through and top with sliced scallion.