Thaw bread dough and let rise according to package instructions.
Dice onion, peppers, and mushrooms. Saute in a skillet with 1 Tablespooon of butter.
Preheat oven to 375 degrees. Line a jellyroll pan with parchment paper or spray with nonstick spray.
Roll bread dough out to a 12" x 18" rectangle. It doesn't have to be perfect!
Mix mayonnaise with garlic powder. Spread over rolled out bread leaving an half inch border.
Add a layer of deli roast beef.
Add the onions, peppers, and mushrooms.
Add a layer of cheese.
Roll the stromboli up jelly-roll style. It will be easier to roll up tightly if you used shredded cheese. I used provolone slices and I folded the edges over the middle. It makes a shorter and wider roll, but that's okay! Set it on a baking sheet.
Melt remaining tablespoon of butter. Brush melted butter over the top of the roll. Sprinkle with parsley, garlic seasoning, and sesame seeds if desired.
Bake 15-20 minutes until golden brown and warmed all the way through.