Grilled Portobello Quesadillas

Name: Grilled Portobello Quesadillas
Ingredients: 3 tablespoons olive oil; 2 tablespoons balsamic vinegar; Salt and pepper; 4 to 5 portobello mushrooms (about 1 pound), trimmed and cleaned; 4 flour tortillas (10-inch); 8 ounces Monterey Jack cheese, shredded (2 1/2 cups); 2 scallions, thinly sliced; Grilled-Tomato Salsa, for serving; Sour cream, for garnish (optional)