In a large mixing bowl, whisk together the olive oil, lemon juice, garlic, mustard, 1½ teaspoons of the salt, and several grinds of pepper.
Place the potatoes in a large pot and cover with cold water by about 1 inch. Stir in the remaining 1 teaspoon salt. Bring to a boil, then reduce the heat and simmer, uncovered, until fork-tender, 10 to 14 minutes. Drain the potatoes, pat dry, and cut any larger potatoes in half. Add the potatoes to the bowl with the dressing and toss to coat.
Refill the pot with water, bring it to a boil, and set a bowl of ice water nearby. Drop the green beans into the boiling water and blanch for 2 minutes, drain, and immediately immerse in the ice water to stop the cooking process. After 1 minute, drain and place on a kitchen towel to dry, then add them to the bowl with the potatoes along with the corn and onion, and toss. Season to taste with more salt and pepper, if desired. Transfer to a platter.
In a small bowl, mix the Vegan Sour Cream with about 2 tablespoons water and stir until it’s a drizzleable consistency. Drizzle over the salad and sprinkle with the chives and red pepper flakes. Season to taste and serve.