Step 1

Add the balsamic vinegar, soy sauce, olive oil, rosemary, and garlic powder to a storage bag or shallow dish. Stir together the marinade. Put the sliced portobello into the marinade and allow them to sit for 15 minutes or up to an hour.

Step 2

Preheat the oven to 350 degrees.

Step 3

While the oven is preheating, add the butter to a large pan. Melt the butter over medium heat, add the portobello mushrooms with the marinade the pan.

Step 4

Stir and cook the mushrooms until tender, about 5-8 minutes.

Step 5

While the mushrooms are cooking, use a sharp, serrated bread knife to cut the King's Hawaiian Rolls in half. Do not separate them into individual rolls. Set the rolls on a baking sheet.

Step 6

Layer half of the shredded cheese on the bread.

Step 7

Use tongs to pull the mushrooms out of the saucy mixture and put a layer of mushrooms on the cheese. You do not want extra sauce in the sandwich.

Step 8

Layer the second half of the cheese on top.

Step 9

Cover the cheese and mushroom mixture with the top of the rolls.

Step 10

Cover and bake for 15-20 minutes or until the buns are toasted and the cheese is melted.

Step 11

Remove from the oven. Following the lines of the rolls, cut the vegetarian sliders into individual sandwiches.