Sweet Potato Spears with Lemongrass Dip

• 3 sweet potato try to find evenly shaped ones, not too twisted or crooked
• olive oil for brushing
• 1 tsp ground coriander
• 1 tsp coriander seeds
• 1 tsp yellow mustard seeds
• 3/4 tsp salt sea salt
• black pepper fresh cracked, to taste
• 1 Serrano pepper thinly sliced, or jalapeño pepper

for the Lemongrass Dip:

• 1 lemongrass 1 stalk
• 3/4 cup crème fraîche Crema Mexicana, sour cream, or Greek yogurt
• lime zest and juice of 1
• 1 inch ginger peeled, piece
• 1/2 tsp salt sea salt