Clean the mussels: Place 4 lb mejillones frescos limpios y sin barbas in a Colander and rinse under cold running water. Scrub any grit off the shells with a stiff brush and pull out any beards (the fibrous threads) by tugging firmly toward the hinge. Discard any 4 lb mejillones frescos limpios y sin barbas that are cracked or remain open after a firm tap on the counter.

Sauté the aromatics: Melt 2 tablespoons of 3 tbsp mantequilla sin sal dividida 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.

Add the wine and steam the mussels: Pour in the 1 ½ cups vino blanco seco como Sauvignon Blanc o Pinot Grigio and bring to a boil over medium-high heat. Add the cleaned 4 lb mejillones frescos limpios y sin barbas, season with 3 tbsp mantequilla sin sal dividida and ¼ tsp kosher salt, and stir to coat. Cover the Pot tightly with a Lid and steam for 5–7 minutes, shaking the pot once or twice, until all the 4 lb mejillones frescos limpios y sin barbas have opened. Discard any 4 lb mejillones frescos limpios y sin barbas that remain closed after cooking.

Finish and serve: Remove the Pot from heat. Stir in the remaining 1 tablespoon of 3 tbsp mantequilla sin sal dividida until melted into the broth. Scatter the chopped ½ cup perejil de hoja plana fresco picado aproximadamente over the 4 lb mejillones frescos limpios y sin barbas 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 Crusty Baguette on the side.