Grilled Everything Salad

Name: Grilled-Everything Salad
Ingredients: 2 tablespoons prepared horseradish; 3 tablespoons red wine vinegar; 5 tablespoons extra-virgin olive oil; 1 teaspoon kosher salt; 1/2 teaspoon freshly ground pepper; 1 10-ounce package (or 2 bunches) fresh spinach, stems removed; 1 1/2 pounds grilled flank steak, thinly sliced (6 cups); 1 large onion, thickly sliced and grilled (3 cups); 3 carrots, thinly sliced and grilled (3 cups); 3 plum tomatoes, halved and grilled