Here's one of my favorites burger sauces:
Feral Sauce (A riff on In-and-Out's Animal Style Sauce)
1 white oninon
2 Tbsp. unsalted butter
2 tsp. + pinch kosher salt, divided
2 tsp. + black pepper, divided
2 Tbsp. + dash Worcestershire sauce, divided
1 cup mayo
1/2 cup ketchup
2 Tbsp finely...