Preheat the oven to 375 degrees. Wash the sweet potatoes and then cut them into desired widths for fries. In a large bowl, toss the cut sweet potatoes with olive oil, salt, and rosemary. Arrange them in a single layer on one or two baking sheets that have been lightly oiled. Bake for about 40 minutes, flipping them once, or until they are the desired degree of browned that you prefer.
For the ketchup: heat the olive oil in a medium skillet. Add the onion and cook, stirring frequently, until the onion has just softened. Put the cooked onion in your blender, along with the rest of the ketchup ingredients. Blend until smooth. You can use the sauce as-is at this point and it will be great, but I like to then pour it into a medium saucepan and let it simmer for ten minutes, just to blend the flavors. This is good served hot or cold.