Preheat oven to 425ºF. Line a 12-cup muffin tin and 4 cups of a 6-cup tin with paper liners. Melt 2 Tbsp. unsalted butter in a skillet. Sauté onion for 3 minutes. Add jalapeño; cook, stirring, 2 minutes. Remove to a plate. Melt 4 Tbsp. unsalted butter in skillet; remove to a bowl. Whisk flour, cornmeal, sugar, baking powder, baking soda and salt. In a separate bowl, whisk eggs, milk, buttermilk and melted butter. Stir milk mixture into flour mixture. Gently fold in pepper Jack and onion mixture. Divide batter among muffin cups and bake until firm, 12 to 14 minutes. Serve warm.