Drain and rinse the split peas and combine them with plenty of purified water and sea salt in a medium pot. Bring to a boil, lower the heat to a simmer and cook, partially covered, for about 30 minutes, until soft, but not mushy. Drain over a colander and set aside.
In the meantime, preheat oven to 400° F (200° C). Prepare two parchment paper-covered baking trays.
Place the potatoes on one of the trays, drizzle with half of the oil, sprinkle with salt, pepper, and half of the rosemary, if using, and mix. Put the carrots and onion on the second tray, do not mix them together. Drizzle both with oil, salt, pepper and the rest of the rosemary, if using, and toss to coat. Place both trays in the oven and roast for 30-40 minutes, until all vegetables are golden and cooked throughout. The onion might cook quicker than the potatoes and carrots. Remove it from the baking sheet earlier, if that's the case.
Let the roasted vegetables cool slightly and combine them with the cooked split peas in a large bowl, add the herbs and the vinaigrette, and toss to coat. This salad gets even better with time, as everything marinates in the vinaigrette.