You might think it would be hard to produce sweet potato fries that are simultaneously burnt and soggy... yet I've managed to perform this feat several times! What am I doing wrong? I coat them in some bacon fat or lard, add a bit of sea salt and paprika and put them in a preheated 350 degree oven. Every time, they burn and go limp rather than getting crisp!
I use a non primal oxymoron microwave and first slice a hole in the yam or sweet potato--to prevent it from exploding
and cook til soft. Then slice 1/4 inch thick while hot and pan fry in butter and coconut oil til golden brown on both sides. It is a great substitute for the hash brown potatos I used to consume in volume.
Sweet potato fries are tricky in that they have a higher moisture content. So you're not going to get the same effect as with regular potatoes.
One option would be to bake them at a low temperature to dry out, and then put them in again at a 350 degree temperature -- sort of like what I used to do with blanching regular fries: blanche them in hot oil at 275, then fry them again at 350.
Another thought is that they might be too close together/crowded. If they are, they're not going to get crisp on you at all. You might have to move them apart from each other, and break up your single batch onto two cookie sheets.
I personally like a combo of bacon grease/butter/splash of EVOO. Sprinkle with mildly ground Himalayan rocksalt and dip in a home-made Garlic All-i-oli (Real Catalan Mayonnaise made with NO egg yolk :)
P.S. Did you know that Mayonnaise comes from Mahón on the Island of Menorca?
grease the baking sheet with butter, cut them into uniform strips, drizzle melted butter on top, bake at 400 degrees for 30 minutes. No flipping or opening the oven. Remove with a metal spatula and season them afterwards.
After years of either soggy or charred sweet potato fries, I stumbled upon a technique that has yet to fail. Like many above mentioned, uniform cut is important. Once cut, toss in a whisked mixture of egg white and olive or coconut oil. Let excess drip off and place on baking sheet with no fries touching. Bake for 20 - 30 minutes at 425F. Check often - SPFs have a tipping point that takes them from crispy to charcoal in the time it takes to pour another glass of red. Season with salt, pepper, whatever while still hot.