Peel and slice sweet potatoes into 1/2″ inch thick rounds. Add them to a large casserole dish and add olive oil and salt. Toss until covered.
In a saucepan, melt butter over medium high heat. Add in sugar, maple syrup, cinnamon, nutmeg, ginger, orange zest, and orange juice. Bring to a boil. Let boil for 2-3 minutes, stirring the entire time, until mixture begins to thicken.
Turn off heat and add vanilla. Pour sauce over top of sweet potatoes and toss until fully coated.
Bake for 30 minutes uncovered, then toss potatoes to full cover in juice.
Cover with foil and bake for an additional 20-25 minutes or until sweet potatoes are fork tender.
Take out of oven and gently scoop some extra juice from the bottom of the pan and run overtop of sweet potatoes.
Top with rosemary and sea salt. Let cool for at least 10 minutes.