In a small bowl, stir the yogurt, tandoori paste, cumin, and lemon juice. Refrigerate the yogurt mix for at least 30 minutes.
Preheat the oven to 375ºF. Line a baking sheet with parchment paper.
Peel the sweet potato and chop it into 1/2-inch cubes. Toss the sweet potatoes with olive oil, and bake for 23-25 minutes, or until the sweet potatoes can be easily pierced with a fork. Take the sweet potatoes out of the oven, and increase the heat to 400 degrees F. Remove the sweet potatoes and the parchment paper. Line the baking sheet with a fresh piece of parchment paper.
Stem the leaves of the collard greens from the stalk. Chop them into 1/2-inch thick ribbons.
Time to assemble the flatbread pizza. Spread the yogurt sauce onto both pieces of naan. Lay the toppings (collard greens, red pepper, sliced red onions, goat cheese, and sweet potatoes) on top of the naan.
Bake the flatbread pizza for 8 to 10 minutes or until the edges of the naan are browned.
Serve immediately and refrigerate any leftovers in an airtight container.