Preheat oven to 425º.
Peel sweet potatoes and slice in half lengthwise. Cut into fry-shape pieces, continuing to slice lengthwise into wedges about 1/3” thick. Try to cut relatively the same thickness and size, so they’ll cook evenly.
Place potatoes in a large plastic ziplock bag. Add coriander, cumin, and fennel seeds. Seal bag and shake to coat. Open, and add olive oil. Shake to coat again, evenly distributing seasoning and oil.
Line a baking sheet with parchment paper, and spread potatoes in a single, even layer. Sprinkle with sea salt.
Bake for 10-12 minutes, then flip potatoes with a metal spatula. Bake another 7-10 minutes, until they begin to brown and blister.
Remove from oven.
While potatoes bake, place tahini, water, lemon juice, garlic, and sea salt in a blender and blend till completely smooth. Then add parsley and cilantro and pulse a few seconds more, until the herbs are flecked throughout.
Serve fries topped with tahini sauce, crumbled feta, and more fresh cilantro and parsley!