Intensify and mix the flavors by grinding the sage, rosemary, and thyme into the salt, rather than just stirring it in. This salt mixture makes a great seasoning also for meats like roasted white fish, grilled chicken, and pork or to take your steamed vegetables to a whole new level.
Preheat the oven to 425°. In a mortar put the sage, rosemary, and thyme. Add the lemon zest and crush it with the pestle. Add the salt and mix until they are finely ground. Reserve them in a small bowl.
In a bowl, roll the potatoes with the oil and butter and season with pepper. With the potatoes in a single layer on large baking sheets, roast for 25 minutes. Season the potatoes generously with the herb salt, toss well and bake for 5 more minutes, or until the potatoes are golden.