1. Line a baking sheet with baking paper. Preheat your oven to 425 degrees Fahrenheit. Bring a large pot of salted water to a boil and boil the potatoes until easily pierced with a knife, around 15 to 20 minutes. Drain and let cool.
2. Meanwhile, make the harissa herb oil. Combine the olive oil, harissa, lemon juice, minced garlic, herbs, salt, and pepper in a small bowl, mixing well to combine. Transfer the cooled potatoes to the prepared baking sheet and use the back of a fish spatula or the bottom of a glass to gently smash the potatoes down, around half an inch thick. Drizzle half of the harissa oil over the smashed potatoes.
3. Place the Cornish hens on the baking sheet over the smashed potatoes and drizzle the remaining harissa oil over the hens. Rub the oil all over the hens and inside the cavities, ensuring they are completely covered. Season with flaky salt and the black pepper.
4. Transfer the baking sheet to the preheated oven and bake for 50 to 60 minutes or until an instant-read thermometer inserted into the thickest part of the breast registers at 165 degrees Fahrenheit.
5. Remove the hens from the oven and let them rest for 20 minutes. Garnish with fresh lemon slices and herbs. Serve.