Step 1Heat the oven to 350 degrees. Brush a deep 12-inch baking dish with the butter. Line with parchment paper.
Step 2Peel the potatoes. Cut lengthwise into quarters, then into half-inch slices. Reserve 1 tablespoon of the oil and heat the remainder in a large skillet. Add the potatoes and fry, turning often, until they are crisp and browned on both sides, about 15 minutes. Drain on paper towels.
Step 3Heat the reserved olive oil in a second large skillet and fry the onion and garlic for about 1 minute. Stir in the peas, olives and harissa. Remove from the heat and set aside.
Step 4Lightly whisk the eggs and cream together in a bowl, then stir in the cilantro and salt.
Step 5Add the potatoes to the onion mixture in the skillet, then add the cream mixture and stir gently to combine. Pour into the prepared baking dish.
Step 6Bake 10 minutes, then reduce the oven setting to 325 degrees and bake 10 minutes longer, or until set. Remove the tortilla from the oven and let stand for 30 minutes.
Step 7Turn upside down onto a serving platter and remove the pan and parchment paper. Spread the tortilla with sour cream. Garnish with the caviar and serve hot or cold.