Preheat oven to 425F. Drizzle oil on sheet pan and preheat in oven until the oil is shimmering.

Add potatoes and baking soda to a medium pot, cover with 3 inches of cold water, and salt heavily. Bring to a boil and cook until potatoes are knife tender, about 15 minutes. Drain and let cool for 5 minutes.

Carefully spread the potatoes on the preheated baking sheet with oil. Bake until golden brown, about 30 minutes, flipping as needed to brown all sides (check every 10 minutes).

Mix the cheese, garlic, herbs, and black pepper in a large bowl.

In a small bowl, mix the mayo, harissa (starting with a little and adding more if desired), and lemon juice. Season with salt and pepper.

Once the potatoes are golden brown, transfer them to the cheese mixture and toss to combine. Spread the harissa mayo on a plate, top with potatoes, and serve immediately.