Preheat the oven to 400° F.
Slice the peeled sweet potatoes into roughly 1-inch thick rounds.
In a small bowl, whisk together white miso paste and maple syrup; gradually add water until the mixture is smooth and free of clumps.
Heat a heavy bottomed, oven-safe skillet over medium-high heat and add avocado oil.
When the oil is shimmering, sear sweet potato rounds in a single layer for 3-4 minutes until browned.
Flip the sweet potatoes with tongs and sear an additional 1-2 minutes on the other side.
Turn off the heat; add butter to the skillet and swirl to melt it.
Pour the miso mixture over the sweet potatoes, lifting each piece to coat underneath.
Transfer the skillet to the oven and bake for 15 minutes.
Remove from oven, use a spoon and oven mitts to baste the potatoes with sauce, then return to bake for another 5-10 minutes until tender.
Transfer sweet potatoes to a serving plate, pour remaining sauce over, scrape up caramelized bits, and sprinkle with flaky sea salt before serving.