Preheat the oven to 400 degrees. Roast potatoes until tender, about 1 hour and 30 minutes.
Let the potatoes cool, then peel and break them in half. Weigh the potatoes.
For every pound of cooked potatoes, use 1 cup of flour and one whisked egg. Rice the potatoes evenly on a work area.
Sprinkle salt, 3/4 of the flour, and the egg over the potatoes. Work the dough gently until formed into a ball.
Cut the dough into 4 pieces, roll into ropes, and cut into gnocchi shapes using a gnocchi board or fork. Transfer to a baking sheet.
Bring a pot of salted water to a boil and cook the gnocchi until they float to the top, indicating they're done.
In a skillet, melt butter and add chopped tomatoes, salt, and pepper. Cook for 2-3 minutes.
Add cooked gnocchi, gnocchi cooking liquid, and minced rosemary to the skillet. Saute for another 2-3 minutes.
Sprinkle grated parmesan cheese over the dish, then serve.