Preheat the oven to 400°F/200°C.

Cook the Potatoes: In a large bowl, toss the potatoes in the 1⁄4 cup (60 ml) olive oil and season with the thyme and about 2 tablespoons salt. Arrange the potatoes on a baking sheet, and bake until the center of the potatoes is tender when pierced with the tip of a knife, 25 to 30 minutes. Remove from the oven and set aside to cool.

Prepare the Cream: In a small bowl, whisk the sour cream with 1 tablespoon warm water until smooth. Season it with 2 teaspoons salt. In another small bowl, whisk together the remaining 2 tablespoons olive oil, the lemon zest, and lemon juice. Mix half of the olive oil mixture into the sour cream. Reserve the rest.

Assemble the Potatoes: Put the potatoes on a flat surface and cut them in half lengthwise. Slice off a bit of each rounded underside so the potato halves sit flat. Arrange them on a serving platter, and top each one with a little of the sour cream and roe. Drizzle with the remaining olive oil mixture. Sprinkle with the scallions and Maldon salt.