Slice off the 4 long sides of each of the sweet potatoes so the potatoes are squared off and sit flat on a cutting board. Slice the potatoes lengthwise into 1/2-inch-thick planks, about 5-inches-by-2-inches (you should get 3 planks per sweet potato). Combine the sweet potatoes, olive oil and a pinch of salt in a medium bowl and toss gently to coat. Spread them out on a baking sheet and roast, flipping halfway through, until lightly browned and tender when pierced with a knife, about 15 minutes. Remove the sweet potato toasts to 2 plates.
Divide the avocado slices among the toasts and sprinkle with a pinch of salt. Top with the radish slices, then sprinkle with the chives, Cotija cheese, red pepper flakes and alfalfa sprouts. Drizzle lightly with olive oil and the lime juice. Serve immediately.
You can make the toasts in advance and refrigerate them in an airtight container up to 4 days. Reheat in a toaster until warmed through.
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