In a large skillet over medium-high heat, cook ground beef and sausage until the meat is no longer pink, breaking up the meat with a wooden spoon as it cooks. Drain off excess grease.
Add the Velveeta cheese to the skillet with the meat. Cook, stirring constantly, over medium-low heat until the cheese melts. Set aside.
Line a deep 11 x 7-inch baking dish with parchment paper, allowing some of the parchment to overhang the sides like handles. This is optional, but the parchment paper “sling” makes it easier to remove the sliders after baking. Place the bottom half of the rolls in the prepared dish.
Assemble the sandwiches by spooning the meat mixture onto the bottom of the rolls. Place the top half of the rolls on top of the meat.
In a small skillet, melt butter over medium heat. Add onion; cook and stir 1-2 minutes or until tender.
Whisk in the brown sugar, Worcestershire sauce, and garlic powder. Continue whisking until brown sugar is dissolved.
Pour butter sauce over sandwiches. Sprinkle with poppy seeds.
Cover with aluminum foil and refrigerate for at least 1 hour, but not more than 4 hours.
Preheat oven to 350°F (180°C). Bake, covered, 25 minutes.
Remove foil and bake uncovered for an additional 5 minutes or until golden brown. Let the sliders rest in the pan for about 10 minutes. Lift the sliders out of the pan onto a serving board, or serve them directly from the dish. Cut the sliders and pull them apart.