Hot Strip Steak Sandwiches

Name: Hot Strip Steak Sandwiches
Ingredients: 1 cup Pinot Noir; 1/2 cup beef or veal demiglace (see Note); 1 cup Niçoise olives, pitted; Salt and freshly ground pepper; 1/4 cup extra-virgin olive oil; 1 large Spanish onion, thinly sliced lengthwise; 1 large red bell pepper, cored and thinly sliced lengthwise; 1 large green bell pepper, cored and thinly sliced lengthwise; 2 pounds strip steak, sliced 1/8 inch thick across the grain; 2 large baguettes, cut in half; 4 cups baby spinach (5 ounces)