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.

Sauté the aromatics: Melt 2 tablespoons of 3 tbsp unsalted butter divided in a Pot over medium heat. Add the 3 medium shallots finely minced and cook, stirring, for 2–3 minutes until softened and translucent. Add the 4 cloves garlic minced and ¼ tsp red pepper flakes (if using) and cook for 1 minute more until fragrant.

Finish and serve: Remove the Pot from heat. Stir in the remaining 1 tablespoon of 3 tbsp unsalted butter divided until melted into the broth. Scatter the chopped ½ cup fresh flat-leaf parsley roughly chopped over the 4 lb fresh mussels scrubbed and debearded and give everything a gentle toss. Ladle into deep bowls, making sure each serving gets plenty of broth. Serve immediately with 1 whole lemon cut into wedges for serving wedges and 1 whole crusty baguette or sourdough bread sliced, for dipping on the side.