Searing some of the veggies over high heat until lightly charred gives the stock alluringly complex flavor and a hint of smokiness. Simmering the mixture no longer than 25 minutes keeps the vegetable flavor fresh and vibrant.
Heat a large stockpot over high heat. Add canola oil to pan; swirl to coat. Add carrots, celery, leeks, onions, and turnip to pan; cook, stirring occasionally, 3 minutes or until browned and slightly charred. Add water; bring to a boil. Reduce heat to low. Add cabbage leaves, broccoli stems, and kale stems; simmer 25 minutes. Strain; cool.