Place the tomatoes, cut sides up, on a rack set on a baking sheet.
Sprinkle with sea salt and place in the oven for about 2 hours.
The tomatoes will dehydrate and intensify in flavor but should still be a bit soft.
Remove from oven and set aside.
Meanwhile, pick over the lentils and discard any misshappen lentils and stones.
Rinse the lentils.
Place in a bowl and add water to cover generously.
Let stand for 1 hour.
Drain and set aside.
In a saucepan over medium heat, warm the olive oil.
Add the onion, carrot, and celery and saute until golden brown, 5-7 minutes.
Add the stock, drained lentils, and potatoes and bring to a boil.
Reduce the heat to low and simmer, uncovered, until the lentils and potatoes are tender, 30-40 minutes.
Add the reserved tomatoes and the rosemary and continue to simmer, stirring gently to retain the shape of the tomatoes, until the flavors have blended, about 15 minutes more.