Preheat the oven to 400 degrees F. Line a sheet pan with aluminum foil.
Place the rosemary, thyme, lemon zest, and fleur de sel in the bowl of a mini food processor and pulse 10 to 12 times, until finely chopped but not puréed. Spread the mixture in a shallow bowl or plate.
Scrub the potatoes, dry them well, and pierce them all over with the tines of a fork. Place the potatoes on the prepared sheet pan and rub them all over with olive oil. Roll each potato in the salt and herb mixture and put them back on the sheet pan. Bake for 60 to 75 minutes, until tender. Slit the top of each potato, squeeze the ends together to open the potato, and top with a generous scoop of whipped feta. Sprinkle with chives and serve hot with extra whipped feta on the side.
Place the feta and cream cheese in the bowl of a food processor fitted with the steel blade and pulse until the cheeses are mixed. Add the olive oil, lemon juice, 1/2 teaspoon salt, and 1/4 teaspoon pepper and process until smooth. Refrigerate until ready to serve.