Preheat the oven to 425 degrees F.
On a parchment-lined baking sheet, toss the carrots, leek, onion, olive oil and salt until combined. Arrange in an even layer and roast in the oven until lightly browned and caramelized, about 30 minutes.
Transfer the vegetables to a large stockpot along with the potato and water (or stock). Bring to a boil; turn the heat to medium and simmer, uncovered, until the potatoes are fork tender, about 20 minutes. Off the heat, add the miso and lemon juice.
Puree using a stand or immersion blender until smooth. Taste for seasoning and add more salt as necessary. (Depending on how salty your miso paste is, you might need more).
Transfer the soup to bowls or bugs and garnish with the black sesame seeds, pepitas or cilantro, if using.