1. Cook first 3 ingredients in 1 teaspoon hot oil in a medium skillet over medium-high heat, stirring occasionally, 5 minutes or until crisp-tender. Sprinkle with salt and pepper to taste.
2. Top half of pizza crust with roast beef. Spoon onion mixture over roast beef; top with cheese slices. Fold crust over filling, and brush both sides of crust with remaining 1 teaspoon olive oil.
3. Cook in a preheated panini press 2 minutes or until golden brown. Cut into 4 wedges, and serve immediately.
Note: If you don't have a panini press, heat a large nonstick skillet over medium-high heat 1 minute. Reduce heat to medium, and place sandwich in hot skillet. Place a heavy skillet on top of sandwich, and cook 2 minutes on each side or until golden brown.