Basic Vegetable Stock

Name: Basic Vegetable Stock
Ingredients: 1 tablespoon olive or vegetable oil; 1 large onion; 2 large carrots; 2 celery ribs, including a few leaves; 4 mushrooms; 1 bunch scallions, including half of the greens; 8 garlic cloves, smashed; 1 tablespoon tomato paste; 8 parsley sprigs with stems; 6 thyme sprigs or ½ teaspoon dried thyme; 2 bay leaves; 2 teaspoons salt; ½ teaspoon peppercorns; 2 quarts cold water