Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry produce. 

Slice sweet potatoes into ½-inch-thick rounds. Halve, peel, and cut onion into ½-inch-thick wedges.

In a small bowl, combine sour cream with half the chimichurri. 

In a large bowl, toss sweet potatoes and onion with a drizzle of oil, half the Fry Seasoning (all for 4 servings), a pinch of salt, and pepper. 

Spread out sweet potatoes and onion on one half of a foil-lined baking sheet (for 4, spread out across entire baking sheet). Roast on top rack for 10 minutes (you’ll add the barramundi then).  

Once veggies have roasted 10 minutes, remove from oven and add a drizzle of oil to opposite side of sheet. Carefully add coated barramundi, skin sides down; return to top rack and roast until veggies are browned and tender and barramundi is cooked through, 10-15 minutes. (For 4 servings, leave veggies roasting; add barramundi to a second oiled baking sheet and roast on middle rack.)

Divide barramundi and veggies between plates. Drizzle with remaining chimichurri and serve.