Preheat the oven to 450 degrees F.
In a small bowl, toss together 1 rounded teaspoon of salt, garam masala, cumin and some freshly ground black pepper to taste.
Add the sweet potatoes to a large bowl, drizzle with enough olive oil to coat, and sprinkle with the spice rub. Toss well to coat evenly.
Pour the sweet potatoes out onto a parchment paper-lined baking sheet or a baking rack, making sure all the pieces are evenly spaced. Bake 30 to 35 minutes, flipping halfway through. Remove the potatoes from the baking sheet and serve immediately.
Combine the cinnamon sticks, cloves, green cardamom seeds, black cardamom seeds, if using, in a spice grinder or coffee grinder and grind until fine. Store the spice mix in an airtight container away from direct sunlight.