Preheat the oven to 425°F. Spray two rimmed baking sheets with nonstick spray or line with parchment paper.
In a small bowl, mix the sage, rosemary, thyme, lemon zest and salt. Use a wooden spoon to press the herbs into the salt mixture. Keep pressing and mixing until flavors are melded together. (You can do this in a mini-food processor if you have one).
In a large bowl, toss the potatoes with the oil and butter and season with pepper. Spread the potatoes in a single layer on the prepared baking sheets. Roast for 25 minutes.
Remove the baking sheets from the oven and sprinkle the potatoes with the herb salt. Toss well and return the potatoes to the oven for an additional 5 to 10 minutes, or until potatoes are tender and golden. Transfer to a bowl and serve warm.