open-faced saucy philly cheesesteak sandwich

Ingredients

• 1 teaspoon olive oil
• 1 1/2 cups red bell pepper strips
• 1 1/2 cups yellow bell pepper strips
• 1 1/2 cups vertically sliced onion
• 1 tablespoon balsamic vinegar
• 1 teaspoon sugar
• 1 teaspoon dried oregano
• 1/4 teaspoon garlic powder
• 4 - (2-ounce) slices bread, toasted
• 3/4 pound thinly sliced deli roast beef
• 2 tablespoons chopped fresh parsley
• 8 ounces shredded cheddar cheese