Hot ham and cheese sandwiches with onion butter

Ingredients

• 8 large or 16 small rolls
• 1/2 pound ham
• 1/2 pound cheese
• 8 tablespoons salted butter
• 1/2 cup finely minced onion
• 3 tablespoons prepared mustard
• 1 teaspoon poppy seeds
• 1 teaspoon freshly squeezed lemon juice