Peel and slice onions to about 1/8-inch thick — thin enough to cook quickly, thick enough to avoid scorching. Yellow or Spanish onions work best. Limit to 2 large onions per 12-inch Skillet to prevent steaming. Have your fat ready and keep a splash of water, broth, or wine nearby to deglaze if needed.

Heat a Skillet over medium to medium-high heat. Add butter, oil, or a mix and let it shimmer. Add sliced onions in large handfuls, stirring for 1–2 minutes per batch until they begin to soften — this prevents overcrowding. Once all onions are in, season with a pinch of salt. Cook over medium heat, stirring every few minutes, for 8–12 minutes total until soft, translucent, and lightly golden. To speed up softening, add 1/4 to ½ cup ½ cup water or broth and cover; steam wilts the onions in a few minutes. Remove the lid once liquid evaporates and continue cooking uncovered. If the pan looks dry, deglaze with a splash of ½ cup water, broth, or wine, scraping up any browned bits. Don't crank the heat — high heat burns the outside before the onions cook through.

Peel and slice onions to about 1/8-inch thick — thin enough to cook quickly, thick enough to avoid scorching. Yellow or Spanish onions work best. Limit to 2 large onions per 12-inch Skillet to prevent steaming. Have your fat ready and keep a splash of water, broth, or wine nearby to deglaze if needed.

Heat a Skillet over medium to medium-high heat. Add butter, oil, or a mix and let it shimmer. Add sliced onions in large handfuls, stirring for 1–2 minutes per batch until they begin to soften — this prevents overcrowding. Once all onions are in, season with a pinch of salt. Cook over medium heat, stirring every few minutes, for 8–12 minutes total until soft, translucent, and lightly golden. To speed up softening, add 1/4 to ½ cup water or broth and cover; steam wilts the onions in a few minutes. Remove the lid once liquid evaporates and continue cooking uncovered. If the pan looks dry, deglaze with a splash of water, broth, or wine, scraping up any browned bits. Don't crank the heat — high heat burns the outside before the onions cook through.

Heat the ⅓ cup vegetable oil or any neutral high-heat oil in a Wok or Skillet over high heat until shimmering. Working in two batches, add the ⅓ cup cornstarch for coating the beef-coated 1 ½ lb flank steak sliced very thin against the grain, about 1/4 inch thick in a single layer. Cook undisturbed for 1–2 minutes until the bottom is crispy and browned, then flip and cook another 30–60 seconds. Transfer to a paper towel-lined plate. Repeat with the remaining 1 ½ lb flank steak sliced very thin against the grain, about 1/4 inch thick.