Lightened Up Shepherds Pie

Name: Lightened-Up Shepherd's Pie
Ingredients: 1 pound all-purpose potatoes, peeled and quartered; 1 medium head cauliflower (about 2 pounds), cored and cut into 1-inch florets; 1 bunch scallions, finely chopped; 1 1/4 cups low-sodium chicken broth; Kosher salt and freshly ground black pepper; 1 pound lean ground beef; 1 teaspoon extra-virgin olive oil; 3 carrots, halved lengthwise and cut into thin half-moons; 3 cloves garlic, finely chopped; 2 stalks celery, finely chopped; 2 medium onions, finely chopped; 2 tablespoons all-purpose flour; 2 tablespoons tomato paste; 2 tablespoons Worcestershire sauce; One 10-ounce package frozen peas, thawed