Coarsely shred the potatoes, using the grating disk in a food processor. (Don’t wash out the food processor—you’ll be using it again right away.) Transfer the potatoes to a colander or strainer and use your hands or a wooden spoon to press out as much moisture as possible.
Remove the grating disk from the processor and replace with the steel blade. Return about one-third of the shredded potatoes to the food processor. Add the garlic and rosemary and process, using the pulse motion, until roughly pureed. Transfer the mixture to a large bowl. Add the remaining shredded potatoes, the egg, matzoh meal, salt, pepper, and baking powder to the bowl. Mix until thoroughly combined. Refrigerate for about 15 minutes to mingle the flavors.
In a heavy, 10- to 12-inch skillet (cast iron is ideal), heat about ¼inch of oil over high heat until hot but not smoking. Drop ¼cup of the potato latke batter into the pan and flatten with a spatula. Repeat with more batter, cooking no more than 4 or 5 latkes at a time; crowding the pan will give you soggy latkes.
Regulate the heat carefully, reducing it to medium as the latkes fry until golden and crisp on the bottom, about 4 minutes. To prevent oil from splattering, use two spatulas (or a spatula and a large spoon) to turn the latkes carefully. Fry until crisp and golden on the other side.
It’s best to flip the latkes only once, so that they don’t absorb too much oil. So, before turning, lift the latkes slightly with the spatula to make sure the underside is crisp and brown.
As the latkes are done, transfer them to paper towels or untreated brown paper bags to drain.
Continue making latkes in the same manner until all the batter is used. If necessary, add more oil to the pan, but always allow the oil to get hot before frying a new batch.
Serve straightaway. Or keep the latkes warm in a 200°F oven (place in a single layer on an ovenproof platter lined with paper towels) and serve when they are all ready to be brought to the table.