Triple-Threat Onion Galette

Heat 3 Tbsp. unsalted butter in a medium skillet over medium until foaming, about 1 minute. Reduce heat to medium-low; add onion, garlic, and 1 tsp. Diamond Crystal or ½ tsp. Morton salt, and stir to combine. Cover and cook, stirring occasionally, until onion is tender and just beginning to brown, 6–8 minutes. Uncover and continue to cook, stirring occasionally and adding a splash of water if onion begins to stick, until golden brown in spots, about 5 minutes more. Transfer onion mixture to a plate; reserve skillet.