Melt butter in a skillet over medium heat. Add onion and garlic; cook and stir until onion is tender, 5 to 7 minutes.

Preheat a waffle iron according to manufacturer's instructions.

Combine onion mixture, mashed potatoes, eggs, flour, salt, and black pepper in a large bowl until well blended.

Scoop 1/4 to 1/2 cup batter (depending on size of waffle iron) into the center of waffle iron and close lid. Cook until golden brown, 3 to 5 minutes.