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 4 tbsp unsalted butter divided, 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.

Reduce the heat to medium. Add the remaining 2 tablespoons of Butter to the same Skillet. Once melted, add the Minced Garlic (and Red Pepper Flakes if using) and sauté for about 1 minute, stirring constantly, until fragrant and just golden — don't let it burn. Pour in the White Wine and Lemon Juice, scraping up any browned bits from the bottom of the pan. Let the sauce simmer for 2–3 minutes until slightly reduced.

Return the Shrimp to the Skillet and toss to coat in the Garlic Butter sauce. Cook for 1 more minute just to warm through. Taste and adjust seasoning with Salt and Black Pepper. Remove from heat, scatter fresh Italian Flat-Leaf Parsley over the top, and serve immediately with Lemon wedges on the side.