How to Make It
Preheat grill to medium-high (about 450°F). Coat onion slice with cooking spray; grill, uncovered, until charred and tender, 3 to 4 minutes per side. Transfer to a small bowl, and cover with plastic wrap. Let stand until fully tender, about 10 minutes. Keep grill hot.
Meanwhile, bring beer to a boil in a large Dutch oven over medium-high; add corn, and return to a boil. Remove from heat; cover and let stand until corn is mostly tender, 5 to 8 minutes. Drain.
Finely chop onion slice. Stir together onion, butter, mustard, salt, and pepper. Shape into a 4-inch log; wrap in plastic wrap, and chill until ready to use.
Grill corn, uncovered, turning often, until charred and fully tender, 5 to 6 minutes. Drizzle hot corn with vinegar, and serve with butter mixture.