Melt the butter (or heat the olive oil) in a saucepan over a medium heat. Add the leeks and a pinch of salt and saute, stirring occasionally, until the leeks have softened and are beginning to color (~10 minutes).
While the leeks are cooking, wash and peel the sweet potato and parsnips. Grate on the coarse holes of a box grater, and place in a large bowl (if you have no patience for hand-grating, you can use the shredding disk on a food processor, but place about ¼ of the mixture back in the bowl of the processor with the regular blade and pulse a few times). Add the salt, pepper, nutmeg, feta, egg and matzo meal. Stir to combine. Mix in the cooked leeks.
Pour the canola oil to a depth of ½” in a frying pan – you can use the pan used for the leeks, and additional pans if you’d like to make quick work of it. Heat the oil over a medium flame until hot – if you drop in a shred of the latke mixture, it should bubble vigorously. Shape ~3 tablespoons of the latke mixture into a round shape (I like to pack a ¼ cup measure ¾ full), and place in the oil. Flatten slightly to form a small pancake. Repeat as many times as your pan space allows. Cook the latkes until well-browned, ~5-7 minutes, then flip and brown the other side. These latkes are more delicate than standard potato pancakes (especially when warm), so be delicate. When the second side has cooked, place on a plate lined with brown paper, stacking as needed. If you want to be extra-good, now and then strain out any rogue bits that have floated into the oil before they burn (or leave them in, for a taste closer to what grandma would have made). Serve. Or...
If you’re not serving at once, layer the cooled latkes in a sealed container with parchment between the layers, and freeze. To serve, preheat the oven to 375 degrees. Place the latkes on a cookie sheet (you can place them on a rack on top of a cookie sheet for a crisper result, but usually the sheet is fine for me), and cook until they have colored a bit more and are heated through and sizzling (~10-15 minutes).