My experince Nady, is almost any way I cook the orange "yam" type sweet potato it has that mushy texture. Even as a steak fry (thick wedge, olive oil, into oven for 45 min at 375F) the outside is crisp, the inside is mushy. I love them so this isnt an issue for me. But, I'm well aware of the texture b/c my daughter has texture issues and turns her nose at these yet, she'll gobble up regular potatoes cooked the same way like there is no tomorrow.
Next time, try the white, sweet potato, it's texture holds up and is our preferred choice at home. The Japanese (purple one) is pretty decent too but, has a tougher skin, if you decide to keep the skin on and eat it.
Oooh if you are gonna try the mandolin sliced and fried in oil, that might be your best bet!
definitely cooked in oil and ghee, with a good amount of salt over them when you first put them in.