1 tbsp. olive oil
Cook onions with salt and olive oil until they start to brown, 6-10 minutes.
Add beef broth, scraping up brown bits and let simmer until the liquid is almost evaporated, 10-20 minutes.
Reduce heat to medium-low and cook until onions are a deep golden brown 5-8 minutes. Stir in onion powder and vinegar and cook until evaporated 1-2 minutes.
Remove from heat and let cool 20 minutes.
Combine sour cream and yogurt. Stir in onion mixture and chill 30 minutes.
Note: Can cover and refrigerate the onion mixture (step 1) for up to 3 days or freeze for up to 2 months.