Heat 3 tablespoons olive oil in a large saute pan over medium heat until shimmering, then add the sliced onions.
Cook onions, stirring often, until soft and beginning to brown- about 20 minutes.
Preheat the oven to 400*F and line a sheet pan with parchment paper or foil.
Spread the sliced sweet potatoes onto the sheet pan in an even layer. Drizzle with olive oil and sprinkle with Kosher salt.
Place the pan in the oven and back 12-15 minutes or until the sweet potatoes have browned and are softened all the way through. Remove and set aside until serving.
While the onions and sweet potatoes are cooking, cook the steak.
Add the herb blend ingredients to the blender or food processor. Blend, pausing occasionally to scrape the sides, and adding more olive oil if necessary to make a thick, combined paste.
Preheat a cast iron pan over high heat until smoking.
Spread half the herb blend on one side of the steak.
Place the steak, herb blend down, on the pan. Spread the remaining herb blend on top.
Cook the steak, turning occasionally, until it’s cooked to your desired degree of doneness.
Remove from the pan and serve with sweet potatoes and onions.