Soak clams in salted water for 10–15 minutes to release sand. Rinse well.
Heat a wide skillet over medium-high. Add clams and pour in beer (Modelo works well).
Stir together clamato, soy sauce, Maggi, Worcestershire, black pepper, and lime. Pour into the pan until liquid reaches halfway up the clams.
Bring to a simmer, then add sliced jalapeño, Korean chili flakes (optional), and another squeeze of lime.
Finish in a hot oven or cover and steam on the stovetop until clams open and broth reduces.
Remove from heat, add butter, and swirl to create a glossy, cohesive sauce.
Rim a skillet with lime and tajín. Add clams and broth, then finish with cilantro and freshly grated Parmesan. Serve with butter-toasted sourdough for dipping.
