Place the 3 peeled, halved potatoes in a medium–large pot and add cold water to cover them by about 1 inch. Bring to a boil over high heat.
Reduce heat to medium and simmer until the potatoes are tender when pierced with a fork, about 20 minutes. Drain and let the potatoes cool until they are cool enough to handle.
Zest the 1 lemon, then juice it and measure 2 teaspoons of lemon juice.
In a bowl, stir together 1/2 cup low‑fat Greek yogurt, 1 cup low‑fat ricotta, the lemon zest, 2 teaspoons lemon juice, 1/3 teaspoon salt, and 1/2 teaspoon poppy seeds until smooth and evenly combined.
Arrange the halved potatoes cut‑side up and spoon the lemon poppy ricotta mixture on top of each potato half. Serve.