Place the cubed russet potatoes in a large bowl of cold water, and the sweet potatoes and onion in another bowl of water.
Soak for 20 minutes until the russet water becomes cloudy.
Preheat the oven to 400F.
Drain the russet potatoes and dry them thoroughly with the paper towels.
Toss the dried russets with 2 tablespoons of the avocado oil and half of the seasonings in a large bowl.
Spread the russets in a single layer on a large rimmed baking sheet and roast for 20 minutes.
Meanwhile, drain and thoroughly dry the sweet potatoes and onion.
Toss them with the remaining oil and seasonings.
After 20 minutes, remove the pan and add the sweet potato-onion mixture, tossing everything together on the hot pan.
Return to the oven and roast for 25-30 minutes more, tossing every 10-15 minutes.
Cook until the russets are golden and crispy outside, fluffy inside, and the sweet potatoes are tender with caramelized edges.
Taste and adjust salt if needed, then serve immediately while hot.