• 4 bunches flat-leaf spinach, trimmed, rinsed, with some water clinging
• 6 tablespoons (3/4 stick) unsalted butter
• 2 teaspoons (or more) red wine vinegar
• Kosher salt, freshly ground pepper

Gradually add spinach to a large dry pot over medium heat, allowing it to wilt between additions; cook until tender, 5-8 minutes. Drain; press to remove liquid. Wipe out pot.

Melt butter in same pot over medium heat. Add spinach and 2 teaspoons vinegar. Season with salt, pepper, and more vinegar, if desired; toss to coat.