Preheat the oven to 450°F with a rack at the center position. Line a large, shallow heavy-duty baking pan with aluminum foil. Place the onions, celery, carrots, fennel, leeks, green pepper, and garlic on the lined baking pan and drizzle with the vegetable oil. Toss the vegetables to coat them with the oil and roast them in the oven for 30 minutes. Remove the pan from the oven to stir the vegetables after the first 15 minutes.

While the vegetables are roasting, place the parsley, thyme, bay leaf, peppercorns, and fennel fronds in a piece of cheesecloth and tie securely. When the vegetables are lightly browned around the edges, scrape them with any accumulated juices into the insert if the slow cooker. Bury the cheesecloth bag among the vegetables, pour in the water, and add the salt. Cover and cook on HIGH for 6 hours, until the stock is bubbling, the vegetables are very soft, and the stock is a deep amber color.

Turn the slow cooker off and let the stock cool slightly. Strain the stock into a bowl. Press hard on the vegetables and cheesecloth bag, then discard them. Store the stock in covered containers in the refrigerator for 1 week, or freeze for up to 6 months.