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Export 7 ingredients for grocery delivery
Step 1
Position racks in top and bottom thirds of the oven and heat oven to 400 degrees.
Step 2
Melt 1 tablespoon butter in a large skillet over medium heat and cook onion, stirring occasionally, until translucent, 8 to 10 minutes.
Step 3
As the onion cooks, split each flat of rolls in half, running a bread knife parallel to the counter as you slice the still-connected rolls. Once the onion has finished cooking, remove from heat and set aside. Place two sheet pans in the oven, one on each rack, to heat while assembling the rolls.
Step 4
In a large mixing bowl, combine mustard, poppy seeds and Worcestershire sauce. Add the remaining cup of butter and warm onion. Stir in cheese and ham until evenly combined.
Step 5
On a sheet of aluminum foil large enough to completely encase a flat of rolls, place one flat, and open like a book. Spread about a quarter of the ham mixture in an even layer over the bottom half of the rolls to the edges. Close the book, so to speak, laying the still-connected roll top over the ham-covered base. Wrap the rolls tightly with the foil, crimping the edges of the foil to enclose them. Repeat with the three other flats of party rolls.
Step 6
Divide the four packets between the two heated sheet pans. Bake until the cheese melts and rolls get toasty, 10 to 12 minutes. Unwrap and serve the flats on platters, encouraging diners to pull a bun from its brethren.