In a small bowl, whisk together the buttermilk, mayonnaise, sriracha sauce and salt until combined.
The Potatoes:
Bring a large saucepan of lightly salted water to a boil over high heat. Add sweet and russet potatoes and cook until just tender, about 5 to 8 minutes. Drain.
Heat the olive oil in a large skillet (preferably cast-iron) set over medium-high heat.
Add garlic and cook for 1 minute, stirring frequently.
Stir in the cooked sweet and russet potatoes and salt. Cook, stirring occasionally, for 5 to 7 minutes.
Add the endive and green onions and cook until the potatoes are starting to brown, 3 to 4 minutes.
Stir in the cilantro. Divide between 6 plates and drizzle with sriracha buttermilk sauce.