Warm the oil and butter in a large skillet over medium heat.
Add the onions and a pinch of salt. Cook, stirring often, until deep golden and jammy, 20–25 minutes. Splash in water as needed to prevent scorching.
Transfer onions to a plate and let cool until just warm.
In a mixing bowl, whisk sour cream, mayo, onion powder, garlic powder, lemon juice, tamari, remaining salt, pepper, and sugar until smooth.
Fold in the cooled onions and chives. Taste and adjust seasoning.
Cover and chill at least 30 minutes for flavors to meld. Garnish with extra chives before serving.