Preheat oven to 450°. In a large bowl, combine sweet potato rounds, 2 tbsp. oil, the fennel seeds, salt, and chile flakes. Toss to coat. Transfer sweet potatoes to a parchment paper-lined rimmed baking sheet and spread out in a single layer. Bake, turning once, until soft and golden, about 20 minutes.
Meanwhile, make salsa verde: Mix garlic, capers, parsley, zest, and remaining 1/4 cup oil.
Heat baguette pieces in the oven until just warmed through, about 8 minutes; then make 4 sandwiches with sweet potato rounds, salsa verde, and feta.