Preheat oven to 350-degrees.
In a large bowl, whisk together the eggs, milk, salt, pepper, 2 teaspoons minced onion, and the ground mustard. Set aside.
Cook the breakfast sausage in a large skillet until cooked through and browned. Remove to a bowl.
In the same large skillet, cook and scramble the eggs. Set aside.
Place the bottom half of the Hawaiian rolls in a casserole dish or baking sheet.
Top the bottom half of the rolls evenly with the sliced cheese.
Evenly spoon the scrambled eggs over the sliced cheese and then top the scrambled eggs with the cooked and crumbled breakfast sausage.
Top the cooked sausage evenly with the shredded cheese.
Place the top half of the rolls on the shredded cheese, pressing lightly.
In a small bowl, stir together the melted butter, 1 teaspoon minced onion, and 1 teaspoon poppy seeds.
Brush the seasoned melted butter liberally over the rolls.
Bake the sliders, uncovered, in a preheated oven for 20 minutes.
Remove and serve hot.